Preemptive multitasking employing software emulation of directed exceptions in a multithreading processor

ABSTRACT

A multiprocessor computer system includes an exception domain having multiple thread contexts (TCs) each having a restart address register, and a timer that generates a periodic interrupt request to the exception domain. The exception domain selects an eligible TC to service the interrupt request, which is non-specific regarding which TC to select. A first interrupt handler executes on the selected TC to service the interrupt request to schedule a set of processes assigned by the SMP OS for execution on the selected TC, and write an address of a second interrupt handler to the restart address register of each TC other than the selected TC. The second interrupt handler schedules a plurality of sets of processes assigned by the SMP OS for execution on respective ones of the TCs other than the selected TC.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation-in-part (CIP) of the followingco-pending Non-Provisional U.S. Patent Application, which is herebyincorporated by reference in its entirety for all purposes: Serial No.(Docket No.) Filing Date Title 10/929,097 Aug. 27, 2005 APPARATUS,METHOD, (MIPS.0194-00-US) AND INSTRUCTION FOR SOFTWARE MANAGEMENT OFMULTIPLE COMPUTATIONAL CONTEXTS IN A MULTITHREADED MICROPROCESSOR

The above co-pending Non-Provisional U.S. Patent Application is acontinuation-in-part (CIP) of the following co-pending Non-ProvisionalU.S. Patent Applications, which are hereby incorporated by reference intheir entirety for all purposes: Serial No. (Docket No.) Filing DateTitle 10/684,350 Oct. 10, 2003 MECHANISMS FOR (MIPS.0188-01-US) ASSURINGQUALITY OF SERVICE FOR PROGRAMS EXECUTING ON A MULTITHREADED PROCESSOR10/684,348 Oct. 10, 2003 INTEGRATED MECHANISM (MIPS.0189-00-US) FORSUSPENSION AND DEALLOCATION OF COMPUTATIONAL THREADS OF EXECUTION IN APROCESSOR

The above two co-pending Non-Provisional U.S. Patent Applications claimthe benefit of the following expired U.S. Provisional Applications,which are hereby incorporated by reference in their entirety for allpurposes: Serial No. (Docket No.) Filing Date Title 60/499,180 Aug. 28,2003 MULTITHREADING (MIPS.0188-00-US) APPLICATION SPECIFIC EXTENSION60/502,358 Sep. 12, 2003 MULTITHREADING (MIPS.0188-02-US) APPLICATIONSPECIFIC EXTENSION TO A PROCESSOR ARCHITECTURE 60/502,359 Sep. 12, 2003MULTITHREADING (MIPS.0188-03-US) APPLICATION SPECIFIC EXTENSION TO APROCESSOR ARCHITECTURE

FIELD OF THE INVENTION

The present invention relates in general to the field of multithreadedmicroprocessors, and particularly to a multiprocessor operating systemdirecting inter-processor interrupts thereon.

BACKGROUND OF THE INVENTION

Microprocessor designers employ many techniques to increasemicroprocessor performance. Most microprocessors operate using a clocksignal running at a fixed frequency. Each clock cycle the circuits ofthe microprocessor perform their respective functions. According toHennessy and Patterson (see Computer Architecture: A QuantitativeApproach, 3rd Edition), the true measure of a microprocessor'sperformance is the time required to execute a program or collection ofprograms. From this perspective, the performance of a microprocessor isa function of its clock frequency, the average number of clock cyclesrequired to execute an instruction (or alternately stated, the averagenumber of instructions executed per clock cycle), and the number ofinstructions executed in the program or collection of programs.Semiconductor scientists and engineers are continually making itpossible for microprocessors to run at faster clock frequencies, chieflyby reducing transistor size, resulting in faster switching times. Thenumber of instructions executed is largely fixed by the task to beperformed by the program, although it is also affected by theinstruction set architecture of the microprocessor. Large performanceincreases have been realized by architectural and organizational notionsthat improve the instructions per clock cycle, in particular by notionsof parallelism.

One notion of parallelism that has improved the clock frequency ofmicroprocessors is pipelining, which overlaps execution of multipleinstructions within pipeline stages of the microprocessor. In an idealsituation, each clock cycle one instruction moves down the pipeline to anew stage, which performs a different function on the instruction. Thus,although each individual instruction takes multiple clock cycles tocomplete, the multiple cycles of the individual instructions overlap.Because the circuitry of each individual pipeline stage is only requiredto perform a small function relative to the sum of the functionsrequired to be performed by a non-pipelined processor, the clock cycleof the pipelined processor may be reduced. The performance improvementsof pipelining may be realized to the extent that the instructions in theprogram permit it, namely to the extent that an instruction does notdepend upon its predecessors in order to execute and can thereforeexecute in parallel with its predecessors, which is commonly referred toas instruction-level parallelism. Another way in which instruction-levelparallelism is exploited by contemporary microprocessors is the issuingof multiple instructions for execution per clock cycle. Thesemicroprocessors are commonly referred to as superscalar microprocessors.

What has been discussed above pertains to parallelism at the individualinstruction-level. However, the performance improvement that may beachieved through exploitation of instruction-level parallelism islimited. Various constraints imposed by limited instruction-levelparallelism and other performance-constraining issues have recentlyrenewed an interest in exploiting parallelism at the level of blocks, orsequences, or streams of instructions, commonly referred to asthread-level parallelism. A thread is simply a sequence, or stream, ofprogram instructions. A multithreaded microprocessor concurrentlyexecutes multiple threads according to some scheduling policy thatdictates the fetching and issuing of instructions of the variousthreads, such as interleaved, blocked, or simultaneous multithreading. Amultithreaded microprocessor typically allows the multiple threads toshare the functional units of the microprocessor (e.g., instructionfetch and decode units, caches, branch prediction units, and load/store,integer, floating-point, SIMD, etc. execution units) in a concurrentfashion. However, multithreaded microprocessors include multiple sets ofresources, or contexts, for storing the unique state of each thread,such as multiple program counters and general purpose register sets, tofacilitate the ability to quickly switch between threads to fetch andissue instructions. In other words, because each thread context has itsown program counter and general purpose register set, the multithreadingmicroprocessor does not have to save and restore these resources whenswitching between threads, thereby potentially reducing the averagenumber of clock cycles per instruction.

One example of a performance-constraining issue addressed bymultithreading microprocessors is the fact that accesses to memoryoutside the microprocessor that must be performed due to a cache misstypically have a relatively long latency. It is common for the memoryaccess time of a contemporary microprocessor-based computer system to bebetween one and two orders of magnitude greater than the cache hitaccess time. Instructions dependent upon the data missing in the cacheare stalled in the pipeline waiting for the data to come from memory.Consequently, some or all of the pipeline stages of a single-threadedmicroprocessor may be idle performing no useful work for many clockcycles. Multithreaded microprocessors may solve this problem by issuinginstructions from other threads during the memory fetch latency, therebyenabling the pipeline stages to make forward progress performing usefulwork, somewhat analogously to, but at a finer level of granularity than,an operating system performing a task switch on a page fault. Otherexamples of performance-constraining issues addressed by multithreadingmicroprocessors are pipeline stalls and their accompanying idle cyclesdue to a data dependence; or due to a long latency instruction such as adivide instruction, floating-point instruction, or the like; or due to alimited hardware resource conflict. Again, the ability of amultithreaded microprocessor to issue instructions from independentthreads to pipeline stages that would otherwise be idle maysignificantly reduce the time required to execute the program orcollection of programs comprising the threads.

Multiprocessing is a technique related to multithreading that exploitsthread-level parallelism, albeit at a higher system level, to execute aprogram or collection of programs faster. In a conventionalmultiprocessor system, multiple processors, or CPUs, share a memorysystem and I/O devices. A multiprocessor (MP) operating systemfacilitates the simultaneous execution of a program or collection ofprograms on the multiprocessor system. For example, the system mayinclude multiple Pentium IV processors all sharing a memory and I/Osubsystem running an MP operating system—such as Linux SMP, anMP-capable version of Windows, Sun Solaris, etc., and executing one ormore application programs concurrently.

Multithreading microprocessors exploit thread-level parallelism at aneven lower level than multiprocessor systems by sharing instructionfetch, issue, and execution resources, as described above, in additionto sharing a memory system and I/O devices. An MP operating system mayrun on a multithreading microprocessor if the multithreadingmicroprocessor presents multiple processors, or CPUs, in an architectedmanner recognized by the MP operating system. Perhaps the most highlypublicized example is the Hyper-Threading (HT) Technology employed inthe Intel® Xeon® multithreading microprocessor. An HT Xeon includeseffectively the same execution resources (e.g., caches, execution units,branch predictors) as a non-HT Xeon processor, but replicates thearchitectural state to present multiple distinct logical processors toan MP OS. That is, the MP operating system recognizes each logicalprocessor as a separate processor, or CPU, each presenting thearchitecture of a single processor. The cost of replicating thearchitectural state for the additional logical processor in the Xeon interms of additional chip size and power consumption is almost 5%.

One aspect of the architecture presented by each of the multipleprocessors to the MP operating system is the ability to handle a list ofarchitected exceptions. Generally speaking, an exception is an error orother unusual condition or event that occurs during the execution of aprogram. In response to an exception, the processor saves the state ofthe currently executing program and begins fetching and executinginstructions at a predetermined address, thereby transferring executionto an alternate program, commonly referred to as an exception handlerlocated at the predefined address. The predefined address may be commonto all exceptions in the list of architected exception types or may beunique to some or all of the exception types. The exception handler,when appropriate, may restore the state and resume execution of thepreviously executing program. Examples of common exceptions include apage fault, a divide by zero, a faulty address generated by the program,a bus error encountered by the processor when attempting to read amemory location, or an invalid instruction exception caused by aninvalid instruction opcode or invalid instruction operand.

Another common exception type is an interrupt, or interrupt request.Interrupts are typically grouped as hardware interrupts and softwareinterrupts. A software interrupt is generated when the currentlyexecuting program executes an architected software interruptinstruction, which causes an exception that transfers control to thearchitected interrupt vector associated with the software interrupt toinvoke an interrupt service routine, or handler. A hardware interrupt isa signal received by the processor from a device to request service bythe processor. Examples of interrupting devices are disk drives, directmemory access controllers, and timers. In response to the interruptrequest, the processor transfers control to an architected interruptvector associated with the interrupt request to invoke an interruptservice routine, or handler.

One function which MP operating systems need to be able to perform isfor one processor, or CPU, to interrupt the operation of anotherspecific one of the processors, and in some cases to all the processorsin the system. These operations are sometimes referred to asinter-processor interrupts (IPIs). Commonly in a multiprocessor system,each processor includes an interrupt controller, which enables eachprocessor to direct an interrupt specifically to each of the otherprocessors. The HT Xeon processors, for example, include a replicatedAdvanced Programmable Interrupt Controller (APIC) for each logicalprocessor, which enables each logical processor to send a hardwareinterrupt specifically to each of the other logical processors.

An example of the use of an IPI is in preemptive time-sharing operatingsystems, which receive periodic timer interrupts, in response to whichthe operating system may perform a task switch on one or more of theprocessors to schedule a different task or process to execute on theprocessors. In Linux SMP, for example, the timer handling routinerunning on the processor that receives the timer interrupt not onlyschedules the tasks on its own processor, but also directs an interruptto each of the other processors to cause them to schedule their tasks.Each processor has an architected interrupt mechanism, which the timerinterrupt-receiving processor uses to direct an IPI to each of the otherprocessors in the multiprocessor system.

Another multithreading microprocessor core architecture which takes asomewhat different approach than, for example, the Intel HT architectureis the MIPS® Multithreading (MT) Application-Specific Extension (ASE) tothe MIPS32® Architecture. The MIPS MT ASE allows two distinct, but notmutually-exclusive, multithreading capabilities. A single MIPS MT ASEmicroprocessor core comprises one or more Virtual Processing Elements(VPEs), and each VPE comprises one or more thread contexts (TCs). Thisarchitecture is described in the document MIPS32® Architecture forProgrammers Volume IV-f: The MIPS® MT Application-Specific Extension(ASE) to the MIPS32® Architecture, Document Number: MD00378, Revision1.00, Sep. 28, 2005, available from MIPS Technologies, 1225 CharlestonRoad, Mountain View, Calif. 94043-1353, which is hereby incorporated byreference in its entirety for all purposes. Embodiments of thearchitecture are also described in the above-referenced U.S. PatentApplications.

In the MIPS MT ASE architecture, an N-VPE processor core presents to anSMP operating system an N-way symmetric multiprocessor. In particular,it presents to the SMP operating system N MIPS32® Architectureprocessors. Thus, SMP operating systems configured to run on aconventional multiprocessor system having N MIPS32 processors withoutthe MT ASE capability will run on a single MIPS32 core with the MT ASEcapabilities with little or no modifications to the SMP operatingsystem. In particular, each VPE presents an architected exception domainto the SMP operating system including an architected list of exceptionsthat the VPE will handle. The list includes interrupts that one VPE maydirect to another specific VPE in the multithreading microprocessor,somewhat similar to the HT Xeon approach.

As mentioned above, each VPE comprises at least one thread context, andmay comprise multiple thread contexts. A thread context in the MIPS MTASE comprises a program counter representation, a set of general purposeregisters, a set of multiplier result registers, and some of the MIPSPrivileged Resource Architecture (PRA) Coprocessor 0 state, such asstate describing the execution privilege level and address spaceidentifier (ASID) of each thread context. The thread contexts arerelatively lightweight compared to VPEs with respect to storage elementsrequired to store state and are therefore less expensive than VPEs interms of chip area and power consumption. Advantageously, thelightweight feature of MIPS MT ASE thread contexts makes them inherentlymore scalable than VPEs, and potentially than Intel HT logicalprocessors, for example.

In particular, in the interest of providing lightweight thread contextsand the concomitant advantages, such as improved scalability, within theMIPS MT ASE, the domain for exception handling is at the VPE level, notthe thread context level. In particular, a VPE handles asynchronousexceptions, such as interrupts, opportunistically. That is, when anasynchronous exception is raised to the VPE, the VPE selects one of theeligible (i.e., not marked as exempt from servicing asynchronousexceptions) thread contexts to execute the exception handler. Thus,although there is an architected means for a thread context to direct anasynchronous exception to a VPE, the thread context cannot specify tothe VPE which thread context should handle the exception within the VPEin a MIPS MT ASE processor, i.e., the exception architecture does notprovide an explicit way for the thread context to direct an asynchronousexception to a specific other thread context. This is a problem,particularly with MP operating systems, such as Linux SMP, that rely onthe ability of one CPU to direct an inter-processor interrupt to anotherCPU in response to a timer interrupt request in order to accomplishpreemptive multitasked process scheduling.

Therefore, what is needed is a system and method for accomplishingpreemptive multitasking by directing an asynchronous timer interruptrequest from one thread context to another specific thread context on amultithreading microprocessor in which such an operation is notarchitecturally supported.

BRIEF SUMMARY OF INVENTION

The present invention provides a system and method for an operatingsystem to perform preemptive multitasking by employing softwareemulation of the operation of a first thread context to direct anexception to another specified thread context in a multithreadingmicroprocessor whose architecture does not explicitly support such anoperation. The multithreading microprocessor includes architectedinstructions that enable one thread context to directly read and writethe other thread contexts. The microprocessor includes an exceptiondomain that receives a timer interrupt request. The exception domainselects one of its eligible thread contexts to service the timerinterrupt request. The exception domain causes the selected threadcontext to execute a first interrupt handler that performs processscheduling as necessary for a set of processes assigned by the MP OS toitself, and then directs an emulated inter-processor interrupt to eachof the other thread contexts of the microprocessor using the architectedinstructions that enable one thread context to read and write anotherthread context.

In one aspect, the present invention provides a method for performingpreemptive scheduling of a plurality of processes managed by a symmetricmultiprocessor operating system on a multithreading microprocessor. Themethod includes an exception domain receiving a periodic interruptrequest from a timer. The exception domain includes a plurality ofthread contexts of the microprocessor. The method also includesexecuting a first interrupt handler on one of the plurality of threadcontexts to service the interrupt request, which includes scheduling anew process to execute on the one thread context and writing an addressof a second interrupt handler to a restart address register of eachthread context other than the one of the plurality of thread contexts.The method also includes the second interrupt handler scheduling arespective new process to execute on each thread context other than theone of the plurality of thread contexts.

In another aspect, the present invention provides a multiprocessorcomputer system for performing preemptive scheduling of a plurality ofprocesses managed by a symmetric multiprocessor operating system (SMPOS) running on the system. The system includes a multithreadingmicroprocessor that includes an exception domain. The exception domainhas a plurality of thread contexts each comprising a restart addressregister. The system also includes a timer that generates a periodicinterrupt request to the exception domain. The exception domain selectsan eligible one of the plurality of thread contexts to service theinterrupt request. The timer interrupt request is non-specific regardingwhich of the plurality of thread contexts to select to service therequest. The system also includes a memory, coupled to themicroprocessor, which stores a first interrupt handler of the SMP OS forexecution on the selected thread context to service the interruptrequest. The first interrupt handler schedules a set of processesassigned by the SMP OS for execution on the selected thread context andwrites an address of a second interrupt handler to the restart addressregister of each of the plurality of thread contexts other than theselected thread context. The second interrupt handler schedules aplurality of sets of processes assigned by the SMP OS for execution onrespective ones of the plurality of thread contexts other than theselected thread context.

In another aspect, the present invention provides a computer programproduct for use with a computing device. The computer program productincludes a computer usable medium, having computer readable program codeembodied in the medium, for causing a method for performing preemptivescheduling of a plurality of processes managed by a symmetricmultiprocessor operating system on a multithreading microprocessor. Thecomputer readable program code includes first program code for providinga step of an exception domain receiving a periodic interrupt requestfrom a timer. The exception domain includes a plurality of threadcontexts of the microprocessor. The computer readable program code alsoincludes second program code for providing a step of executing a firstinterrupt handler on one of the plurality of thread contexts to servicethe interrupt request to schedule a new process to execute on the onethread context and to write an address of a second interrupt handler toa restart address register of each thread context other than the one ofthe plurality of thread contexts. The computer readable program codeincludes third program code for providing a step of the second interrupthandler scheduling a respective new process to execute on each threadcontext other than the one of the plurality of thread contexts.

In another aspect, the present invention provides a method for providingoperating system software for performing preemptive scheduling of aplurality of processes managed by a symmetric multiprocessor operatingsystem on a multithreading microprocessor. The method includes providingcomputer-readable program code describing the operating system software.The program code includes first program code for providing a step of anexception domain receiving a periodic interrupt request from a timer.The exception domain includes a plurality of thread contexts of themicroprocessor. The program code also includes second program code forproviding a step of executing a first interrupt handler on one of theplurality of thread contexts to service the interrupt request, includingscheduling a new process to execute on the one thread context andwriting an address of a second interrupt handler to a restart addressregister of each thread context other than the one of the plurality ofthread contexts. The program code also includes third program code forproviding a step of the second interrupt handler scheduling a respectivenew process to execute on each thread context other than the one of theplurality of thread contexts. The method also includes transmitting thecomputer-readable program code as a computer data signal on a network.

An advantage of the present invention is that it enables SMP operatingsystems that perform preemptive multitasking to enjoy the benefits ofrunning on a multithreading processor that comprises lightweight threadcontexts, which makes the multithreading processor more scalable to arelatively large number of logical processors if needed. In particular,it eliminates the need for an interrupt controller for each threadcontext (e.g., a pending interrupt register and interrupt mask registerand associated control for each thread context); rather, these elementsmay be implemented on a higher level, such as at the VPE level in theMIPS MT ASE, thereby saving chip area and power. Furthermore, it enablesan SMP operating system that performs preemptive multitasking to executeon a microprocessor with potentially many CPUs (i.e., thread contexts),which may yield substantially higher performance than a microprocessorwith a relatively small number of CPUs due to the exploitation ofthread-level parallelism, and yet avoid the additional chip area andpower consumption that would otherwise be required if the per-VPE statewere replicated for each thread context. Finally, the invention may beincorporated into existing SMP operating systems that perform preemptivemultitasking with a relatively small amount of modification to theoperating system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a microprocessor according to thepresent invention.

FIG. 2 is a block diagram illustrating in more detail the microprocessorof FIG. 1.

FIG. 3 is a block diagram illustrating an MFTR instruction executed bythe microprocessor of FIG. 1 according to the present invention.

FIG. 4 is a block diagram illustrating an MTTR instruction executed bythe microprocessor of FIG. 1 according to the present invention.

FIG. 5 is a series of block diagrams illustrating variousmultithreading-related registers of the microprocessor of FIG. 1according to one embodiment of the present invention.

FIG. 6 is a block diagram illustrating data paths of the microprocessorfor performing the MFTR instruction according to the present invention.

FIG. 7 is a block diagram illustrating data paths of the microprocessorfor performing the MTTR instruction according to the present invention.

FIG. 8 is a flowchart illustrating operation of the microprocessor toexecute the MFTR instruction according to the present invention.

FIG. 9 is a flowchart illustrating operation of the microprocessor toexecute the MTTR instruction according to the present invention.

FIG. 10 is a flowchart illustrating a method for performing aninter-processor interrupt (IPI) from one thread context to anotherthread context within a VPE of the microprocessor of FIG. 1 according tothe present invention.

FIG. 11 is a flowchart illustrating a method for performing preemptiveprocess scheduling by a symmetric multiprocessor operating system on themicroprocessor of FIG. 1 according to the present invention.

FIG. 12 is a flowchart illustrating a method for providing software forperforming the steps of FIG. 10 or portions of the source code listingand subsequently transmitting the software as a computer data signalover a communication network.

FIG. 13 is a flowchart illustrating a method for providing software forperforming the steps of FIG. 11 or portions of the source code listingand subsequently transmitting the software as a computer data signalover a communication network.

DETAILED DESCRIPTION

For a better understanding of exception processing on MIPS architectureprocessors in general, the reader is referred to MIPS RISC Architecture,by Gerry Kane and Joe Heinrich, published by Prentice Hall, and to SeeMIPS Run, by Dominic Sweetman, published by Morgan Kaufman Publishers.

Embodiments of the present invention are described herein in the contextof a processor core that includes the MIPS® MT Application-SpecificExtension (ASE) to the MIPS32® Architecture; however, the presentinvention is not limited to a processor core with said architecture.Rather, the present invention may be implemented in any processor systemwhich includes a plurality of thread contexts for concurrently executinga corresponding plurality of threads, but which does not include aninterrupt input for each of the plurality of thread contexts that wouldallow one thread context to direct an inter-processor interruptspecifically to another thread context.

Referring now to FIG. 1, a block diagram illustrating a microprocessor100 according to the present invention is shown. The microprocessor 100includes a virtual multiprocessor (VMP) context 108 and a plurality ofvirtual processing elements (VPEs) 102. Each VPE 102 includes a VPEcontext 106 and at least one thread context (TC) 104. The VMP context108 comprises a collection of storage elements, such as registers orlatches, and/or bits in the storage elements of the microprocessor 100that describe the state of execution of the microprocessor 100. Inparticular, the VMP context 108 stores state related to global resourcesof the microprocessor 100 that are shared among the VPEs 102, such asthe instruction cache 202, instruction fetcher 204, instruction decoder206, instruction issuer 208, instruction scheduler 216, execution units212, and data cache 242 of FIG. 2, or other shared elements of themicroprocessor 100 pipeline described below. In one embodiment, the VMPcontext 108 includes the MVPControl Register 501, MVPConf0 Register 502,and MVPConf1 Register 503 of FIGS. 5B-5D described below.

A thread context 104 comprises a collection of storage elements, such asregisters or latches, and/or bits in the storage elements of themicroprocessor 100 that describe the state of execution of a thread, andwhich enable an operating system to manage the resources of the threadcontext 104. That is, the thread context describes the state of itsrespective thread, which is unique to the thread, rather than stateshared with other threads of execution executing concurrently on themicroprocessor 100. A thread—also referred to herein as a thread ofexecution, or instruction stream—is a sequence of instructions. Themicroprocessor 100 is a multithreading microprocessor. That is, themicroprocessor 100 is configured to concurrently execute multiplethreads of execution. By storing the state of each thread in themultiple thread contexts 104, the microprocessor 100 is configured toquickly switch between threads to fetch and issue instructions. Theelements of a thread context 104 of various embodiments are describedbelow with respect to the remaining Figures. Advantageously, the presentmicroprocessor 100 is configured to execute the MFTR instruction 300 ofFIG. 3 and the MTTR instruction 400 of FIG. 4 for moving thread context104 information between the various thread contexts 104, as described indetail herein.

The VPE context 106 includes a collection of storage elements, such asregisters or latches, and/or bits in the storage elements of themicroprocessor 100 that describe the state of execution of a VPE 102,which enable an operating system to manage the resources of the VPE 102,such as virtual memory, caches, exceptions, and other configuration andstatus information. Consequently, a microprocessor 100 with N VPEs 102may appear to an operating system as an N-way symmetric multiprocessor.However, as also described herein, a microprocessor 100 with M threadcontexts 104 may appear to an operating system as an M-way symmetricmultiprocessor. In particular, threads running on the thread contexts104 may include MFTR instructions 300 and MTTR instructions 400 to readand write another thread context 104 to emulate a directed exception,such as an inter-processor interrupt, as described herein.

The VPEs 102 share various of the microprocessor 100 resources, such asthe instruction cache 202, instruction fetcher 204, instruction decoder206, instruction issuer 208, instruction scheduler 216, execution units212, and data cache 242 of FIG. 2, transparently to the operatingsystem. In one embodiment, each VPE 102 substantially conforms to aMIPS32 or MIPS64 Instruction Set Architecture (ISA) and a MIPSPrivileged Resource Architecture (PRA), and the VPE context 106 includesthe MIPS PRA Coprocessor 0 and system state necessary to describe one ormore instantiations thereof. In one embodiment, the VPE context 106includes the VPEControl Register 504, VPEConf0 Register 505, VPEConf1Register 506, YQMask Register 591, VPESchedule Register 592, andVPEScheFBack Register 593 of FIGS. 5E-5H and EPC Register 598 of FIG. 5Land Status Register 571 of FIG. 5M described below.

In one respect, a VPE 102 may be viewed as an exception domain. That is,when an asynchronous exception (such as a hardware or softwareinterrupt) is generated, or when an instruction of one of the threadcontexts 104 of a VPE 102 generates a synchronous exception (such as anaddress error, bus error, or invalid instruction exception),multithreading is suspended on the VPE 102 (i.e., only instructions ofthe instruction stream associated with the thread context 104 servicingthe exception are fetched and issued), and each VPE context 106 includesthe state necessary to service the exception. Once the exception isserviced, the exception handler may selectively re-enable multithreadingon the VPE 102. When an asynchronous exception such as an interrupt israised to the VPE 102, the VPE 102 selects one of the eligible (i.e.,not marked as exempt from servicing asynchronous exceptions as indicatedby the IXMT bit 518 of FIG. 5J) thread contexts 104 of the VPE 102 toexecute the exception handler. (The manner used by the VPE 102 to selectone of the eligible thread contexts is implementation-dependent, such asselecting pseudo-randomly, in a round-robin fashion, or based on therelative priorities of the thread contexts 104.) That is, theasynchronous exception itself does not specify which thread context 104of the VPE 102 is to handle the exception. Thus, the microprocessor 100does not provide a hardware exception mechanism for one thread context104 to direct an asynchronous exception to another specific threadcontext 104. Advantageously, the present invention provides a method foroperating system software to emulate one thread context 104 directing anasynchronous exception to another specific thread context 104, asdescribed herein.

Referring now to FIG. 2, a block diagram illustrating in more detail themicroprocessor 100 of FIG. 1 is shown. The microprocessor 100 is apipelined microprocessor comprising a plurality of pipeline stages. Themicroprocessor 100 includes a plurality of thread contexts 104 ofFIG. 1. The embodiment of FIG. 2 shows four thread contexts 104;however, it should be understood that the number of four thread contexts104 is chosen only for illustration purposes, and the microprocessor 100described herein embodying the present invention is susceptible to anynumber of thread contexts 104. In one embodiment, the number of threadcontexts 104 may be up to 256. Furthermore, a microprocessor 100 mayinclude multiple VPEs 102, each having multiple thread contexts 104. Inone embodiment, each thread context 104 comprises a program counter (PC)222 for storing an address for fetching a next instruction in theassociated instruction stream, a general purpose register (GPR) set 224for storing intermediate execution results of the instruction streamissuing from the thread context based on the program counter 222 value,and other per-thread context 226. In one embodiment, the microprocessor100 includes a multiplier unit, and the other thread context 226includes registers for storing results of the multiplier unitspecifically associated with multiply instructions in the instructionstream. In one embodiment, the other thread context 226 includesinformation for uniquely identifying each thread context 104. In oneembodiment, the thread identification information includes informationfor specifying the execution privilege level of the associated thread,such as whether the thread is a kernel, supervisor, or user levelthread, such as is stored in the TKSU bits 589 of the TCStatus Register508 of FIG. 5J. In one embodiment, the thread identification informationincludes information for identifying a task or process comprising thethread. In particular, the task identification information may be usedas an address space identifier (ASID) for purposes of translatingphysical addresses into virtual addresses, such as is stored in theTASID bits 528 of the TCStatus Register 508. In one embodiment, theother per-thread context 226 includes the TCStatus Register 508,TCRestart Register 594, TCHalt Register 509, TCContext Register 595,TCSchedule Register 596, TCBind Register 556 and TCScheFBack Register597 of FIGS. 5J-5L.

The microprocessor 100 includes a scheduler 216 for scheduling executionof the various threads being concurrently executed by the microprocessor100. The scheduler 216 is coupled to the VMP context 108 and VPEcontexts 106 of FIG. 1 and to the other per-thread context 226. Inparticular, the scheduler 216 is responsible for scheduling fetching ofinstructions from the program counter 222 of the various thread contexts104 and for scheduling issuing of the fetched instructions to executionunits 212 of the microprocessor 100, as described below. The scheduler216 schedules execution of the threads based on a scheduling policy ofthe microprocessor 100. The scheduling policy may include, but is notlimited to, any of the following scheduling policies. In one embodiment,the scheduler 216 employs a round-robin, or time-division-multiplexed,or interleaved, scheduling policy that allocates a predetermined numberof clock cycles or instruction issue slots to each ready thread in arotating order. The round-robin policy is useful in an application inwhich fairness is important and a minimum quality of service is requiredfor certain threads, such as real-time application program threads. Inone embodiment, the scheduler 216 employs a blocking scheduling policywherein the scheduler 216 continues to schedule fetching and issuing ofa currently running thread until an event occurs that blocks furtherprogress of the thread, such as a cache miss, a branch misprediction, adata dependency, or a long latency instruction. In one embodiment, themicroprocessor 100 comprises a superscalar pipelined microprocessor, andthe scheduler 216 schedules the issue of multiple instructions per clockcycle, and in particular, the issue of instructions from multiplethreads per clock cycle, commonly referred to as simultaneousmultithreading.

The microprocessor 100 includes an instruction cache 202 for cachingprogram instructions fetched from a system memory of a system includingthe microprocessor 100, such as the MFTR/MTTR 300/400 instructions. Inone embodiment, the microprocessor 100 provides virtual memorycapability, and the fetch unit 204 includes a translation lookasidebuffer for caching physical to virtual memory page translations. In oneembodiment, each thread, or program, or task, executing on themicroprocessor 100 is assigned a unique task ID, or address space ID(ASID), which is used to perform memory accesses and in particularmemory address translations, and a thread context 104 also includesstorage for an ASID associated with the thread. In one embodiment, thevarious threads executing on the microprocessor 100 share theinstruction cache 202 and translation lookaside buffer. In anotherembodiment, each thread includes its own translation lookaside buffer.

The microprocessor 100 also includes a fetch unit 204, coupled to theinstruction cache 202, for fetching program instructions, such asMFTR/MTTR 300/400 instructions, from the instruction cache 202 andsystem memory. The fetch unit 204 fetches instructions at an instructionfetch address provided by a multiplexer 244. The multiplexer 244receives a plurality of instruction fetch addresses from thecorresponding plurality of program counters 222. Each of the programcounters 222 stores a current instruction fetch address for a differentprogram thread. The embodiment of FIG. 2 illustrates four differentprogram counters 222 associated with four different threads. Themultiplexer 244 selects one of the four program counters 222 based on aselection input provided by the scheduler 216. In one embodiment, thevarious threads executing on the microprocessor 100 share the fetch unit204.

The microprocessor 100 also includes a decode unit 206, coupled to thefetch unit 204, for decoding program instructions fetched by the fetchunit 204, such as MFTR/MTTR 300/400 instructions. The decode unit 206decodes the opcode, operand, and other fields of the instructions. Inone embodiment, the various threads executing on the microprocessor 100share the decode unit 206.

The microprocessor 100 also includes execution units 212 for executinginstructions. The execution units 212 may include but are not limited toone or more integer units for performing integer arithmetic, Booleanoperations, shift operations, rotate operations, and the like; floatingpoint units for performing floating point operations; load/store unitsfor performing memory accesses and in particular accesses to a datacache 242 coupled to the execution units 212; and a branch resolutionunit for resolving the outcome and target address of branchinstructions. In one embodiment, the data cache 242 includes atranslation lookaside buffer for caching physical to virtual memory pagetranslations. In addition to the operands received from the data cache242, the execution units 212 also receive operands from registers of thegeneral purpose register sets 224. In particular, an execution unit 212receives operands from a register set 224 of the thread context 104allocated to the thread to which the instruction belongs. A multiplexer248 selects operands from the appropriate register set 224 for provisionto the execution units 212. In addition, the multiplexer 248 receivesdata from each of the other per-thread contexts 226 and program counters222, for selective provision to the execution units 212 based on thethread context 104 of the instruction being executed by the executionunit 212. In one embodiment, the various execution units 212 mayconcurrently execute instructions from multiple concurrent threads.

The microprocessor 100 also includes an instruction issue unit 208,coupled to the scheduler 216 and coupled between the decode unit 206 andthe execution units 212, for issuing instructions to the execution units212 as instructed by the scheduler 216 and in response to informationabout the instructions decoded by the decode unit 206. In particular,the instruction issue unit 208 insures that instructions are not issuedto the execution units 212 if they have data dependencies on otherinstructions previously issued to the execution units 212. In oneembodiment, an instruction queue is imposed between the decode unit 206and the instruction issue unit 208 for buffering instructions awaitingissue to the execution units 212 for reducing the likelihood ofstarvation of the execution units 212. In one embodiment, the variousthreads executing on the microprocessor 100 share the instruction issueunit 208.

The microprocessor 100 also includes a write-back unit 214, coupled tothe execution units 212, for writing back results of instructions intothe general purpose register sets 224, program counters 222, and otherthread contexts 226. A demultiplexer 246 receives the instruction resultfrom the write-back unit 214 and stores the instruction result into theappropriate register set 224, program counters 222, and other threadcontexts 226 associated with the instruction's thread. The instructionresults are also provided for storage into the VPE contexts 106 and theVMP context 108.

Referring now to FIG. 3, a block diagram illustrating an MFTRinstruction 300 executed by the microprocessor 100 of FIG. 1 accordingto the present invention is shown. FIG. 3 comprises FIG. 3A illustratingthe format and function of the MFTR instruction 300, and FIG. 3Billustrating a table 350 specifying selection of the MFTR instruction300 source register 324 based on its operand values. The mnemonic forthe MFTR instruction 300 is MFTR rt, rd, u, sel, h as shown. As shown inFIG. 3, the MFTR instruction 300 instructs the microprocessor 100 tocopy the contents of a source register 324 of a target thread context104 to a destination register 322 of an issuing thread context 104.

Bits 11-15 are an rd field 308, which specifies an rd register 322, ordestination register 322, within the general purpose register set 224 ofFIG. 2 of the thread context 104 from which the MFTR instruction 300 isissued, referred to herein as the issuing thread context. In oneembodiment, the destination register 322 is one of 32 general purposeregisters of the MIPS ISA.

Bits 16-20, 6-10, 5, 4, and 0-2 are an rt field 306, rx field 312, ufield 314, h field 316, and sel field 318, respectively, whichcollectively are used to specify a source register 324 of a threadcontext 104 distinct from the issuing thread context, referred to hereinas the target thread context 104. The use of the rt field 306, rx field312, u field 314, h field 316, and sel field 318 to specify the sourceregister 324 is described in detail in table 350 of FIG. 3B.

In one embodiment, the microprocessor 100 includes one or more processorcontrol coprocessors, referred to in the MIPS PRA as Coprocessor 0, orCP0, or Cop0, denoted 602 in FIGS. 6 and 8, which is generally used toperform various microprocessor 100 configuration and control functions,such as cache control, exception control, memory management unitcontrol, and particularly multithreading control and configuration. Asshown in Table 350, a u field 314 value of 0 selects one of the CP0registers as the MFTR instruction 300 source register 324. Table 500 ofFIG. 5A illustrates the particular rt field 306 (or rd 308 in the caseof MTTR 400) and sel field 318 values used to select the variousmultithreading-related CP0 registers. In one embodiment, as shown inTable 350, a u field 314 value of 1 and a sel field 318 value of 0selects one of the general purpose registers 224 of FIG. 2, selected bythe rt field 306 value, as the MFTR instruction 300 source register 324.In one embodiment, the microprocessor 100 includes a digital signalprocessor (DSP) arithmetic unit or multiplier for performing commonDSP-related arithmetic operations, and each thread context 104 includesfour accumulators for storing the TC-specific results of the arithmeticoperations and a DSPControl register of the DSP accumulators, denoted224 in FIGS. 6 and 8. A u field 314 value of 1 and a sel field 318 valueof 1 selects as the MFTR instruction 300 source register 324 one of theDSP accumulator registers or the DSPControl register, selected by the rtfield 306 value, as shown. In one embodiment, the microprocessor 100includes one or more floating point or multimedia coprocessors, referredto in the MIPS PRA as Coprocessor 1, or CP1, or Cop1, denoted 604 inFIGS. 6 and 8. As shown in Table 350, a u field 314 value of 1 and a selfield 318 value of 2 selects as the MFTR instruction 300 source register324 one of the floating point unit data registers (FPR) selected by thert field 306 value; furthermore, a sel field 318 value of 3 selects asthe MFTR instruction 300 source register 324 one of the floating pointunit control registers (FPCR) selected by the rt field 306 value. In oneembodiment, the microprocessor 100 includes one or moreimplementation-specific coprocessors, referred to in the MIPS PRA asCoprocessor 2, or CP2, or Cop2, denoted 606 in FIGS. 6 and 8. As shownin Table 350, a u field 314 value of 1 and a sel field 318 value of 4selects as the MFTR instruction 300 source register 324 one of the CP2data registers (Cop2 Data) selected by the concatenation of the rx field312 value and the rt field 306 value; furthermore, a sel field 318 valueof 5 selects as the MFTR instruction 300 source register 324 one of theCP2 control registers (Cop2 Control) selected by the concatenation ofthe rx field 312 value and the rt field 306 value.

The source register 324 is further specified by a TargTC operand 332.The TargTC 332 operand specifies the target thread context 104containing the source register 324. In one embodiment, the TargTCoperand 332 is stored in the VPEControl Register 504 of FIG. 5E. If thesource register 324 is a per-VPE 102 register, the source register 324is of the VPE 102 to which the target thread context 104 is bound, asspecified by the CurVPE field 558 of the TCBind Register 556 of FIG. 5K.

Referring now to FIG. 4, a block diagram illustrating an MTTRinstruction 400 executed by the microprocessor 100 of FIG. 1 accordingto the present invention is shown. FIG. 4 comprises FIG. 4A illustratingthe format and function of the MTTR instruction 400, and FIG. 4Billustrating a table 450 specifying selection of the MTTR instruction400 destination register 422 based on its operand values. The variousfields of the MTTR instruction 400 are identical to the fields of theMFTR instruction 300, except that the value of the sub-opcode field 404is different, and the use of the rt field 306 and rd field 308 isreversed, i.e., the rt field 306 is used by the MTTR instruction 400 toselect the source register 424 and the rd field 308 is used—along withthe rx 312, u 314, h 316, and sel 318 fields—to select the destinationregister 422 within the thread context 104 specified by the TargTC 332operand, as shown in FIG. 4. As shown in FIG. 4, the MTTR instruction400 instructs the microprocessor 100 to copy the contents of a sourceregister 424 of the issuing thread context 104 to a destination register424 of the target thread context 104.

Referring now to FIG. 5, a series of block diagrams illustrating variousmultithreading-related registers of the microprocessor 100 of FIG. 1according to one embodiment of the present invention is shown. FIG. 5comprises FIG. 5A-5M. In one embodiment, the registers of FIG. 5 arecomprised in CP0 602 of FIGS. 6 and 8, and FIG. 5A is a table 500indicating the particular rt field 306 (or rd 308 in the case of MTTR400) and sel field 318 values used to select the variousmultithreading-related CP0 registers 602. As indicated in table 500,some of the registers are included in the VMP context 108 of FIG. 1(i.e., are per-microprocessor 100 registers), some of the registers areincluded in the VPE contexts 106 of FIG. 1 (i.e., are per-VPE 102registers), and some of the registers are included in the threadcontexts 104 of FIG. 1 (i.e., are per-thread context 104 registers).Most of FIGS. 5B-5M include an illustration of the fields of each of themultithreading registers and a table describing the various fields.Fields of particular relevance are discussed in more detail herein. Eachof the registers illustrated in FIG. 5 of one thread context (i.e., thetarget thread context 104) may be selectively read and/or written byanother thread context 104 (i.e., the issuing thread context 104) thatexecutes an MFTR 300 or MTTR 400 instruction, respectively, dependingupon the readability or writeability of the particular register or bitsthereof.

The EVP bit 513 of FIG. 5B controls whether the microprocessor 100 isexecuting as a virtual multiprocessor, i.e., if multiple VPEs 102 mayconcurrently fetch and issue instructions from distinct threads ofexecution. The PVPE field 524 of FIG. 5C specifies the total number ofVPEs 102, i.e., the total number of VPE contexts 106, instantiated inthe microprocessor 100. In the embodiment of FIG. 5, up to sixteen VPEs102 may be instantiated in the microprocessor 100. The PTC field 525 ofFIG. 5C specifies the total number of thread contexts 104 instantiatedin the microprocessor 100. In the embodiment of FIG. 5, up to 256 threadcontexts 104 may be instantiated in the microprocessor 100. The TE bit543 of FIG. 5E controls whether multithreading is enabled or disabledwithin a VPE 102. In one embodiment, the effect of clearing the EVP bit513 and TE bit 543 may not be instantaneous; consequently the operatingsystem should execute a hazard barrier instruction to insure that allVPEs 102 and thread contexts 104, respectively, have been quiesced.

As discussed above, TargTC field 332 of FIG. 5E is used by an issuingthread context 104 to specify the thread context 104 that contains thesource register 324 in the case of an MFTR instruction 300 or thedestination register 422 in the case of an MTTR instruction 400. In oneembodiment, the issuing thread context 104 executes an instruction priorto the MFTR/MTTR instruction 300/400 to populate the TargTC 332 field ofthe VPEControl Register 504. In one embodiment, a single TargTC 332value per VPE 102 is sufficient since multithreading must be disabled onthe VPE 102 issuing the MFTR/MTTR 300/400 instruction; hence, none ofthe other thread contexts 104 of the VPE 102 may be using the TargTC 332field of the VPEControl Register 504 of the issuing VPE 102. In analternate embodiment, the TargTC 332 value may be provided within afield of the MFTR/MTTR 300/400 instructions. The TargTC field 332 isused to specify the target thread context 104 independent of the VPE 102to which the target thread context 104 is bound. Each thread context 104in the microprocessor 100 has a unique number, or identifier, specifiedin the CurTC field 557 of the TCBind Register 556 of FIG. 5K, withvalues 0 through N-1, where N is the number of instantiated threadcontexts 104, and N may be up to 256. If the target register (sourceregister 324 of an MFTR instruction 300, or destination register 422 ofan MTTR instruction 400) is a per-TC register, then the target registeris in the thread context 104 specified by the TargTC 332 value; if thetarget register is a per-VPE register, then the target register is inthe VPE 102 to which the thread context 104 specified in the TargTC 332is bound.

The TCU0 . . . TCU3 bits 581 of the TCStatus Register 508 of FIG. 5Jcontrol and indicate whether the thread context 104 controls access toits VPE's 102 Coprocessor 0, 1, 2, or 3, respectively. The TCU0 . . .TCU3 bits 581 and TKSU bits 589 of the TCStatus Register 508 correspondto the CU0 . . . CU3 bits 572 and the KSU bits 574, respectively, of theStatus Register 571 of FIG. 5M; and the TASID bits 528 of the TCStatusRegister 508 correspond to the ASID bits of the Coprocessor 0 EntryHiregister described in the MIPS32® Architecture for Programmers VolumeIII: The MIPS32® Privileged Resource Architecture, Document Number:MD00090, Revision 2.50, Jul. 1, 2005, available from MIPS Technologies,1225 Charleston Road, Mountain View, Calif. 94043-1353. In particular,each time the bits are written in one of the registers, thecorresponding change is reflected by a read of the other register. Forexample, if a new value is written to the TKSU bits 589, the new valuemay be read from the KSU bits 574 of the Status Register 571, and viceversa.

The TCContext Register 595 of FIG. 5L is a read/write register usable bythe operating system as a pointer to a thread context-specific storagearea in memory, such as a thread context control block. The TCContextRegister 595 may be used by the operating system, for example, to saveand restore state of a thread context 104 when the program threadassociated with the thread context 104 must be swapped out for use byanother program thread.

The RNST bits 582 of the TCStatus Register 508 indicate the state of thethread context 104, namely whether the thread context 104 is running orblocked, and if blocked the reason for blockage. The RNST 582 value isonly stable when read by an MFTR instruction 300 if the target threadcontext 104 is in a halted state, which is described below; otherwise,the RNST 582 value may change asynchronously and unpredictably. When athread context 104 is in the running state, the microprocessor 100 willfetch and issue instructions from the thread of execution specified bythe thread context 104 program counter 222 according to the scheduler216 scheduling policy.

Independently of whether a thread context 104 is free or activated, athread context 104 may be halted if the H bit 599 of the TCHalt Register509 of FIG. 5K is set. That is, a first thread context 104 running anoperating system thread may halt a second thread context 104 by writinga 1 to the H bit 599 of the TCHalt Register 509 of the second threadcontext 104. A free thread context 104 has no valid content and themicroprocessor 100 does not schedule instructions of a free threadcontext 104 to be fetched or issued. The microprocessor 100 schedulesinstructions of an activated thread context 104 to be fetched and issuedfrom the activated thread context 104 program counter 222. Themicroprocessor 100 schedules only activated thread contexts 104. Themicroprocessor 100 allows the operating system to allocate only freethread contexts 104 to create new threads. Setting the H bit 599 of anactivated thread context 104 causes the thread context 104 to ceasefetching instructions and to load its restart address 549 into theTCRestart register 594 of FIG. 5K with the address of the nextinstruction to be issued for the thread context 104. Only a threadcontext 104 in a halted state is guaranteed to be stable as seen byother thread contexts 104, i.e., when examined by an MFTR instruction300. Multithreaded execution may be temporarily inhibited on a VPE 102due to exceptions or explicit software interventions, but activatedthread contexts 104 that are inhibited in such cases are considered tobe suspended, rather than implicitly halted. A suspended thread context104 is inhibited from any action which might cause exceptions orotherwise change global VPE 102 privileged resource state, but unlike ahalted thread, a suspended thread context 104 may still haveinstructions active in the pipeline; consequently, the suspended threadcontext 104, including general purpose registers 224 values, may stillbe unstable; therefore, the thread context 104 should not be examined byan MFTR instruction 300 until the thread context 104 is halted. In oneembodiment, the effect of clearing the H bit 599 may not beinstantaneous; consequently the operating system should execute a hazardbarrier instruction to insure that the target thread context has beenquiesced.

When a thread context 104 is in a halted state, the TCRestart Register594 may be read to obtain the address 549 of the instruction at whichthe microprocessor 100 will resume execution of the thread context 104when the thread context 104 is restarted. In the case of branch and jumpinstructions with architectural branch delay slots, the restart address549 will advance beyond the address of the branch or jump instructiononly after the instruction in the delay slot has been retired. If thethread context 104 is halted between the execution of a branchinstruction and the associated delay slot instruction, the branch delayslot is indicated by the TDS bit 584 of the TCStatus Register 508.

Conversely, the TCRestart register 594 can be written while its threadcontext 104 is halted to change the address at which the thread context104 will restart. Furthermore, a first thread context 104 running anoperating system thread may restart a second thread context 104 bywriting a 0 to the H bit 599 of the TCHalt Register 509 of the secondthread context 104. Clearing the H bit 599 of an activated threadcontext 104 allows the thread context 104 to be scheduled, and beginfetching and executing instructions at its restart address 549 specifiedin its TCRestart register 594.

In the MIPS PRA, the Coprocessor 0 EPC Register 598 of FIG. 5L containsthe address at which the exception servicing thread context 104 willresume execution after an exception has been serviced and the threadcontext 104 executes an ERET (exception return) instruction. That is,when the thread running on the thread context 104 executes an ERETinstruction, the VPE 102 reads the EPC Register 598 to determine theaddress at which to begin fetching and issuing instructions. Unless theEXL bit 576 of the Status Register 571 of FIG. 5M is already set, themicroprocessor 100 writes the EPC Register 598 when an exception israised. For synchronous exceptions, the microprocessor 100 writes theaddress of the instruction that was the direct cause of the exception,or the address of the immediately preceding branch or jump instruction,if the exception-causing instruction is in a branch delay slot. Forasynchronous exceptions, the microprocessor 100 writes the address ofthe instruction at which execution will be resumed.

In a MIPS MT ASE microprocessor 100, the EPC Register 598 isinstantiated for each VPE 102 in the microprocessor 100. When anexception is raised to a VPE 102, the VPE 102 selects one of its threadcontexts 104 to service the exception. All thread contexts 104 of theVPE 102, other than the thread context 104 selected to service theexception, are stopped and suspended until the EXL bit 576 and ERL bit575 of the Status Register 571 are cleared. When a synchronous exceptionis raised due to the execution of an instruction contained in a threadof execution, the microprocessor 100 selects the thread context 104running the thread containing the offending instruction to service theexception. That is, the general purpose registers 224, program counter222, and other per-thread context 226 of the offending thread context104 are used to service the synchronous exception. When an asynchronousexception is raised, such as an interrupt, the microprocessor 100selects one of the eligible thread contexts 104 bound to the VPE 102 toservice the asynchronous exception. The VPE 102 to which a threadcontext 104 is bound (as indicated by the CurVPE field 558 of the TCBindRegister 556) is the exception domain for the thread context 104. Inparticular, a VPE 102 selects a thread context 104 bound to it, i.e.,within its exception domain, to service an exception. Additionally, athread context 104 utilizes the resources related to handling exceptions(such as the Coprocessor 0 EPC Register 598 and Status Register 571) ofthe exception domain, or VPE 102, to which the thread context 104 isbound when servicing an exception. The method for choosing the eligiblethread context 104 to service an asynchronous exception isimplementation-dependent and may be adapted to satisfy the particularapplication in which the microprocessor 100 is employed. However, asdiscussed herein, the MIPS MT ASE does not provide the capability forthe asynchronous exception to specify which of the thread contexts 104must service the asynchronous exception. The microprocessor 100 savesthe restart address of the thread context 104 selected to service theexception in the EPC Register 598 of the VPE 102 to which the selectedthread context 104 is bound. Additionally, a thread context 104 may bemade ineligible for being selected to service an asynchronous exceptionby setting the IXMT bit 518 in its TCStatus Register 508.

In one embodiment, the program counter 222 of FIG. 2 is not anarchitecturally-visible register, but is affected indirectly by variousevents and instructions. Effectively, the program counter 222 is avirtual program counter represented by various storage elements withinthe microprocessor 100 pipeline, and the meaning or value of the programcounter 222 depends upon the context in which it is examined or updated.For example, as a thread context 104 fetches instructions from theinstruction cache 202, the program counter 222 value is the address atwhich the instructions are being fetched. Thus, in this context thestorage element storing the current fetch address may be viewed as theprogram counter 222. For another example, when an exception is taken andthe VPE 102 selects a thread context 104 to service the exception, theaddress written by the VPE 102 to the EPC Register 598 may be viewed asthe program counter 222 value of the selected thread context 104 in thissituation since when the selected thread context 104 executes an ERETinstruction, fetching for the thread context 104 begins at the EPCRegister 598 value. For another example, the TCRestart register 594 of athread context 104 may be viewed as the program counter 222 when athread context 104 is halted since when the thread context 104 isunhalted, fetching for the thread context 104 begins at the TCRestartregister 594 value.

The Coprocessor 0 Status Register 571 of FIG. 5M is instantiated foreach VPE 102 in the microprocessor 100. Only certain fields of theStatus Register 571 are described herein. For a more detaileddescription of the other bits in the Status Register 571, the reader isreferred to the document MIPS32® Architecture for Programmers VolumeIII: The MIPS32® Privileged Resource Architecture, Document Number:MD00090, Revision 2.50, Jul. 1, 2005, which is hereby incorporated byreference in its entirety for all purposes. As discussed above, the CU0. . . CU3 bits 572 and the KSU bits 574 correspond to the TCU0 . . .TCU3 bits 581 and TKSU bits 589, respectively, of the TCStatus Register508 of FIG. 5J. The ERL bit 575 is set by the microprocessor 100hardware whenever a Reset, Soft Reset, NMI, or Cache Error exception istaken. The EXL bit 576 is set by the microprocessor 100 hardwarewhenever any other exception is taken. When ERL 575 or EXL 576 is set,the VPE 102 is running in kernel mode with interrupts disabled. When theIE bit 577 is set, all interrupts for the VPE 102 are disabled.

Referring now to FIG. 6, a block diagram illustrating data paths of themicroprocessor 100 for performing the MFTR instruction 300 according tothe present invention is shown. The microprocessor 100 includesselection logic 636 that receives the contents of each of the registersof Coprocessor 0 602, Coprocessor 1 604, Coprocessor 2 606, and thegeneral purpose and DSP accumulator registers 224 of FIG. 2 and selectsthe source register 324 contents, which is one of the register contentsfrom the target thread context 104, for provision to deselection logic638 based on values of the rt 306 operand, the rx 312 operand, the u 314operand, the h 316 operand, and the sel 318 operand of the MFTRinstruction 300, as well as the TargTC 332 operand. The deselectionlogic 638 receives the source register 324 contents selected by theselection logic 636 and writes the selected contents into thedestination register 322, which is one of the general purpose registers224 of the issuing thread context 104, based on the value of the rd 308operand of the MFTR instruction 300, as well as signals 632 and 634indicating the issuing VPE 102 and issuing thread context 104,respectively.

Referring now to FIG. 7, a block diagram illustrating data paths of themicroprocessor 100 for performing the MTTR instruction 400 according tothe present invention is shown. The microprocessor 100 includesselection logic 738 that receives the contents of each of the generalpurpose registers 224 of the issuing thread context 104 and selects thesource register 424, which is one of the register contents from theissuing thread context 104, for provision to deselection logic 736 basedon the value of the rt 306 operand of the MTTR instruction 400, as wellas signals 632 and 634 indicating the issuing VPE 102 and issuing threadcontext 104, respectively. The deselection logic 736 receives the sourceregister 424 contents selected by the selection logic 738 and writes theselected contents into the destination register 422, which is one of theregisters of Coprocessor 0 602, Coprocessor 1 604, Coprocessor 2 606, orthe general purpose and DSP accumulator registers 224 of FIG. 2, basedon values of the rd 308 operand, the rx 312 operand, the u 314 operand,the h 316 operand, and the sel 318 operand of the MTTR instruction 400,as well as the TargTC 332 operand. In one embodiment, the selection andde-selection logic of FIGS. 6 and 7 may comprise a hierarchy ofmultiplexers, demultiplexers, data buses, and control logic forgenerating a plurality of bank and register selectors to control themultiplexers and demultiplexers for selecting the appropriate valuesfrom the specified register for provision on the data buses. In oneembodiment, the data paths may also include intermediate registers forstoring the values transferred between the issuing and target threadcontexts over multiple clock cycles.

Referring now to FIG. 8, a flowchart illustrating operation of themicroprocessor 100 to execute the MFTR instruction 300 according to thepresent invention is shown. Flow begins at block 802.

At block 802, the instruction issuer 208 of FIG. 2 issues an MFTRinstruction 300 to the execution units 212. Flow proceeds to decisionblock 803.

At decision block 803, the execution unit 212 examines the TKSU bits 589of the TCStatus Register 508 to determine whether the privilege level ofthe issuing thread context 104 is at kernel privilege level. If so, flowproceeds to decision block 804; otherwise, flow proceeds to block 805.

At block 805, the execution unit 212 raises an exception to the MFTRinstruction 300 since the issuing thread context 104 does not havesufficient privilege level to execute the MFTR instruction 300. Flowends at block 805.

At decision block 804, the execution unit 212 determines whether thetarget thread context 104 is halted by examining the value of the H bit599 of the TCHalt Register 509 of FIG. 5K. If the target thread context104 is halted, flow proceeds to decision block 808; otherwise flowproceeds to block 816.

At decision block 808, the execution unit 212 examines the TargTC 332value of the issuing VPE 102 VPEControl Register 504 to determinewhether the TargTC 332 value is valid. In one embodiment, the TargTC 332value is not valid if the issuing VPE is not the master VPE 102, asindicated by a clear value in the MVP bit 553 of the VPEConf0 Register505 of FIG. 5F. In one embodiment, the TargTC 332 value is not valid ifthe thread context 104 specified by TargTC 332 is not instantiated. Ifthe TargTC 332 value is valid, flow proceeds to decision block 812;otherwise, flow proceeds to block 816.

At decision block 812, the execution unit 212 examines the TCU bits 581in the TCStatus Register 508 of FIG. 5J to determine whether the MFTRinstruction 300 references a coprocessor, and if so, whether thecoprocessor is bound to and accessible by the target thread context 104specified by the TargTC 332 value. If the MFTR instruction 300references a coprocessor, and the coprocessor is not bound to andaccessible by the target thread context 104 specified by the TargTC 332value, flow proceeds to block 816; otherwise, flow proceeds to decisionblock 814.

At decision block 814, the execution unit 212 determines whether thesource register 324 specified by the MFTR instruction 300 isinstantiated. If so, flow proceeds to block 824; otherwise, flowproceeds to block 816.

At block 816, the results of the MFTR instruction 300 are invalid. Thatis, the microprocessor 100 attempts to perform block 824; however, thesource, destination, and values of the data transfer are invalid. Flowends at block 816.

At block 824, the execution unit 212 copies the contents of the sourceregister 324 of the target thread context 104 to the destinationregister 322 of the issuing thread context 104. In one embodiment, themicroprocessor 100, after reading the source register 324, updates thesource register 324 with an update value. In one embodiment, theread/update is performed atomically. In one embodiment, the update valueis provided in the GPR 224 specified by the rd field 308 in the MFTRinstruction 300. Flow ends at block 824.

Referring now to FIG. 9, a flowchart illustrating operation of themicroprocessor 100 to execute the MTTR instruction 400 according to thepresent invention is shown. Flow begins a block 902.

At block 902, the instruction issuer 208 of FIG. 2 issues an MTTRinstruction 400 to the execution units 212. Flow proceeds to decisionblock 903.

At decision block 903, the execution unit 212 examines the TKSU bits 589of the TCStatus Register 508 to determine whether the privilege level ofthe issuing thread context 104 is at kernel privilege level. If so, flowproceeds to decision block 904; otherwise, flow proceeds to block 905.

At block 905, the execution unit 212 raises an exception to the MTTRinstruction 400 since the issuing thread context 104 does not havesufficient privilege level to execute the MTTR instruction 400. Flowends at block 905.

At decision block 904, the execution unit 212 determines whether thetarget thread context 104 is halted by examining the value of the H bit599 of the TCHalt Register 509 of FIG. 5K. If the target thread context104 is halted, flow proceeds to decision block 908; otherwise flowproceeds to block 916.

At decision block 908, the execution unit 212 examines the TargTC 332value of the issuing VPE 102 VPEControl Register 504 to determinewhether the TargTC 332 value is valid. In one embodiment, the TargTC 332value is not valid if the issuing VPE is not the master VPE 102, asindicated by a clear value in the MVP bit 553 of the VPEConf0 Register505 of FIG. 5F. In one embodiment, the TargTC 332 value is not valid ifthe thread context 104 specified by TargTC 332 is not instantiated. Ifthe TargTC 332 value is valid, flow proceeds to decision block 912;otherwise, flow proceeds to block 916.

At decision block 912, the execution unit 212 examines the TCU bits 581in the TCStatus Register 508 of FIG. 5J to determine whether the MTTRinstruction 400 references a coprocessor, and if so, whether thecoprocessor is bound to and accessible by the target thread context 104specified by the TargTC 332 value. If the MTTR instruction 400references a coprocessor, and the coprocessor is not bound to andaccessible by the target thread context 104 specified by the TargTC 332value, flow proceeds to block 916; otherwise, flow proceeds to decisionblock 914.

At decision block 914, the execution unit 212 determines whether thedestination register 422 specified by the MTTR instruction 400 isinstantiated. If so, flow proceeds to block 924; otherwise, flowproceeds to block 916.

At block 916, the microprocessor 100 performs no operation because thereis no valid destination register to which the source data may bewritten. Flow ends at block 916.

At block 924, the execution unit 212 copies the contents of the sourceregister 424 of the issuing thread context 104 to the destinationregister 422 of the target thread context 104. Flow ends at block 924.

Referring now to FIG. 10, a flowchart illustrating a method forperforming an inter-processor interrupt (IPI) from one thread context104 to another thread context 104 within a VPE 102 of the microprocessor100 of FIG. 1 according to the present invention is shown. The steps ofthe flowchart substantially correlate to the source code listing at theend of the Detailed Description, and reference is made within thedescription of FIG. 10 to the source code listing. The source codelisting is for a version of the Linux SMP operating system modified toview each thread context 104 of the microprocessor 100 as a separateprocessor, or CPU, which is referred to herein as symmetric multi-threadcontext (SMTC) Linux. The source code listing includes two C languagefunctions (smtc_send_ipi and post_direct_ipi), one assembly languageroutine (smtc_ipi_vector), and one assembly language macro (CLI).

Within the flowchart, reference is made to a thread A running on athread context A 104 and a thread B running on a thread context B 104.Thread A running on thread context A 104 directs a software-emulatedinter-processor interrupt (IPI) to thread context B 104, by employingMFTR instructions 300 and MTTR instructions 400. In the example of theflowchart, thread context A 104 and thread context B 104 are bound tothe same VPE 102. Although the flowchart of FIG. 10 illustrates only anintra-VPE IPI, the source code listing also includes instructions atlines 23-28 for directing a cross-VPE IPI, or inter-VPE IPI. A firstthread context 104 is said to direct an inter-VPE IPI to a second threadcontext 104 if the second thread context 104 is bound to a different VPE102 than the first thread context 104. The code performs an inter-VPEIPI by placing an IPI message on a queue associated with the targetthread context 104. The message specifies the target thread context 104.In the embodiment described in the source code at lines 23-28, themessage specified the target thread context 104 implicitly by being onthe queue associated with the target thread context 104. The operatingsystem samples the queue and drains it each time the operating systemperforms a context switch and returns from exception. After queuing themessage, the code issues a MIPS PRA asynchronous software interrupt tothe target VPE 102 (i.e., to the VPE 102 to which the target threadcontext 104 is bound) by executing an MTTR instruction 400 (within thewrite_vpe_c0_cause routine) to set one of the software interrupt bits inthe MIPS PRA Cause Register of the target VPE 102, which will cause thequeue to be sampled and drained. If the thread context 104 selected bythe target VPE 102 to service the software interrupt is the target ofthe IPI, then the selected thread context 104 will service the IPIdirectly; otherwise, the selected thread context 104 will direct anintra-VPE IPI to the target thread context 104 in a manner similar tothe operation described in the flowchart of FIG. 10.

As described above, when an asynchronous hardware interrupt (such as aperiodic timer interrupt used for operating system task schedulingpurposes) is requested in a MIPS MT ASE processor, the VPE 102 thatreceived the hardware interrupt request selects an eligible threadcontext (in this example, thread context A 104) to handle the exception.In the MIPS architecture, when a hardware interrupt request is made,control is transferred to a general exception vector of the operatingsystem. The general exception vector decodes the cause of the exceptionand invokes the appropriate interrupt request handler (in this example,thread A), such as the timer handler.

The Linux SMP kernel for the MIPS architecture assumes that everyprocessor, or CPU, in the SMP system will get a periodic interrupt, anddivides the work performed by the timer interrupt handler into a localclock interrupt function that executes on all CPUs, and a system clockinterrupt function that executes only on one CPU of the SMP system. Inthe MIPS processor architecture, each VPE 102 includes one timer inCoprocessor 0 shared by all thread contexts 104 bound to the VPE 102(see the Count/Compare register pairs described in MIPS32® Architecturefor Programmers Volume III: The MIPS32® Privileged ResourceArchitecture, Document Number: MD00090, Revision 2.50, Jul. 1, 2005). Inone embodiment of SMTC Linux, only one of the timers of one of the VPEs102 is invoked as the single timer for all CPUs of the SMP system. Inanother embodiment, the timer of each of the VPEs 102 is invoked for allCPUs of that VPE 102. The thread context 104 selected to service theasynchronous timer interrupt executes the system clock interruptfunction and then broadcasts, or directs, an IPI to all the other threadcontexts 104 of the VPE 102. The directed IPI is a local clock interrupttype IPI which instructs the receiving thread contexts 104 to executeonly the local clock interrupt function. Although the SMTC Linux timerinterrupt handler directs an IPI message to each thread context 104known to the operating system as a processor, the flowchart of FIG. 10only illustrates directing an IPI to one thread context 104, which isthread context B 104 in this example. The operation of themicroprocessor 100 in response to a timer interrupt to performpreemptive task scheduling is described in more detail in FIG. 11. Flowbegins at block 1002.

At block 1002, at source code line 38, thread A running on threadcontext A 104 halts thread B running on thread context B 104 byexecuting an MTTR instruction 400 instruction to clear the H bit 599 ofthe TCHalt Register 509 of FIG. 5K. It is noted that the C languagefunction write_tc_c0_tchalt includes the MTTR instruction 400. Thefunction settc at line 36 populates the TargTC field 332 of theVPEControl Register 504 of FIG. 5E with the thread context 104identifier of the specified thread context 104 (in the example, threadcontext B 104) for the benefit of the MTTR instruction 400 of thewrite_tc_c0_tchalt function. Flow proceeds to block 1004.

At block 1004, at lines 95-100 (via the call the post_direct_ipi at line64), thread A creates a new stack frame on the kernel stack of threadcontext B 104. In one embodiment, the new stack frame is effectivelycreated by the assignment of a value to the kernel stack pointer ofthread context B 104, and storing values on the new stack framecomprises storing values at predetermined offsets from the kernel stackpointer value. It is also noted that if the target thread context 104 isexempted from taking interrupts (as indicated by a set IXMT bit 518 ofFIG. 5J), the code cannot spin waiting for the target thread context 104to become non-exempted from taking interrupts because this may lead to adeadlock condition. Therefore, the code places the IPI message on thetarget thread context's 104 queue at lines 48-62, in a manner similar tothe inter-VPE IPI issued at line 24; however, in this case no inter-VPE102 software interrupt is necessary. Flow proceeds to block 1006.

At block 1006, at line 82, thread A reads the TCStatus Register 508 ofthread context B 104 via the function read_tc_c0_tcstatus, whichincludes an MFTR instruction 300. The TCStatus Register 508 includes thethread context B 104 execution privilege level and interrupt exemptionstatus, among other things. Thread A, at line 104, also saves theTCStatus Register 508 value to the stack frame created at block 1004.Flow proceeds to block 1008.

At block 1008, at line 83, thread A reads the restart address 549 ofthread B from TCRestart register 594 of thread context B 104 via thefunction read_tc_c0_tcrestart, which includes an MFTR instruction 300.Thread A, at line 102, also saves the restart address 549 to the stackframe created at block 1004. Flow proceeds to block 1012.

At block 1012, at lines 106 and 108, thread A saves the address of theoperating system IPI handler and a reference to an IPI message on thestack frame created at block 1004. In the embodiment of the source codelisting, advantageously, the code manipulates the target thread contextB 104 and stack frame such that a common IPI handler may be invoked tosupport SMTC operation. The common IPI handler is invoked to handle bothsoftware emulated interrupts described herein and actual hardwareinterrupts, i.e., interrupts for which target thread context 104 B isthe thread context 104 selected by the VPE 102 to handle the hardwareinterrupt request, such as may be invoked at block 1114 of FIG. 11. Flowproceeds to block 1014.

At block 1014, at lines 110-112, thread A writes the TCStatus Register508 of thread context B 104 via the function the functionwrite_tc_c0_tcstatus, which includes an MTTR instruction 400, to set theexecution privilege level of thread context B 104 to kernel mode anddisables, or exempts, thread context B 104 from receiving interrupts.Conceptually, thread A would set the EXL bit 576 in Coprocessor 0 StatusRegister 571 in order to emulate an exception. However, when EXL 576 isset, multithreading is disables on the VPE 102, i.e., only one threadcontext 104 is allowed to run when EXL 576 is set. And thread A needsthread context B 104 to run when un-halted below at block 1018.Therefore, the setting of EXL 576 must be left up to thread context B104 by smtc_ipi_vector at block 1022 below. Thus, until then, thread Atemporarily accomplishes a similar effect to setting EXL 576 by settingIXMT 518 and TKSU 589 to kernel mode in the thread context B 104TCStatus Register 508. Flow proceeds to block 1016.

At block 1016, at line 115, thread A writes the restart address 549 ofthread B in the TCRestart register 594 of thread context B 104 via thefunction the function write_tc_c0_tcrestart, which includes an MTTRinstruction 400, with the address of smtc_ipi_vector. Flow proceeds toblock 1018.

At block 1018, at line 65, thread A un-halts, or restarts, threadcontext B 104 to cause smtc_ipi_vector to begin running on threadcontext B 104. Flow proceeds to block 1022.

At block 1022, at lines 163-165, the smtc_ipi vector sets EXL 576, whichhas the effect of disabling interrupts and setting the executionprivilege level to kernel mode for all thread contexts 104 bound to theVPE 102. It is noted that at line 160 the smtc_ipi_vector disablesmultithreading on the VPE 102 before setting EXL 576. Additionally, ifmultithreading was enabled prior to line 160, the code restoresmultithreading at lines 168-170. It is also noted that if thread contextB 104 was in user mode when halted at block 1002, the smtc_ipi_vectorsets the CUO bit 572 of the Status Register 571. Flow proceeds to block1024.

At block 1024, at lines 196 and 198, the smtc_ipi_vector restores thethread context B 104 pre-halted TCStatus Register 508 value that wassaved at block 1006, and in particular restores its execution privilegelevel and interrupt exemption state. Flow proceeds to block 1026.

At block 1026, at lines 200-201, the smtc_ipi_vector loads the EPCRegister 598 with the thread context B 104 pre-halted TCRestart register594 value saved at block 1008. Consequently, when the standard Linux SMPreturn from interrupt code subsequently executes an ERET instruction atblock 1036, thread B will be restarted on thread context B 104 at theaddress at which it was halted at block 1002. Thus, by setting EXL 576at block 1022 and populating the EPC Register 598 at block 1026, thesmtc_ipi_vector effectively emulates what the microprocessor 100hardware would do if thread context B 104 had been selected to servicethe asynchronous interrupt (rather than thread context A 104). Flowproceeds to block 1028.

At block 1028, at line 203, the smtc_ipi_vector saves all of the generalpurpose registers 224 to the stack frame created at block 1004. Flowproceeds to block 1032.

At block 1032, at line 204 via the CLI macro, the smtc_ipi_vector setsitself to kernel mode execution privilege level and exempts itself fromservicing interrupts. It is noted that this is performed only for threadcontext B 104, not for the entire VPE 102. It is noted that the CLImacro is a standard Linux macro which is modified to support SMTC bysetting kernel mode execution privilege level and exempting frominterrupt servicing (via the IXMT bit 518) only the invoking threadcontext 104, rather than the entire VPE 102 (as the non-SMTC code doesby clearing the IE bit 577 of the Status Register 571 of FIG. 5M), asshown at lines 227-247. Flow proceeds to block 1034.

At block 1034, at lines 205-210, the smtc_ipi_vector calls the commonIPI handler (which is ipi_decode, as populated at line 108) with the IPImessage reference saved on the stack frame at block 1012 as an argument.Flow proceeds to block 1036.

At block 1036, at line 212, after the operating system IPI handlerreturns, the smtc_ipi_vector jumps to the standard operating systemreturn from interrupt code (which in Linux SMP is ret_from_irq), whicheventually executes an ERET instruction to return execution on threadcontext B 104 to thread B with its pre-halted execution privilege leveland interrupt exemption state. Prior to executing the ERET instruction,the return from interrupt code restores the EPC Register 598 with therestart address value saved at block 1008 and restores the StatusRegister 571 KSU bits 574 with the value saved at block 1006. Flow endsat block 1036.

Referring now to FIG. 11, a flowchart illustrating a method forperforming preemptive process scheduling by a symmetric multiprocessoroperating system (SMP OS), such as Linux SMP, on the microprocessor 100of FIG. 1 according to the present invention is shown. Symmetricmultiprocessor operating systems manage a plurality of processes, ortasks, and assign the execution of the processes to particularprocessors, or CPUs, of the symmetric multiprocessor system, which arethread contexts 104 in the case of microprocessor 100. Within the set ofprocesses assigned to execute on a given CPU, or thread context 104, thepreemptive SMP OS schedules the set of processes to run on the assignedthread context 104 in some time-multiplexed fashion according to thescheduling algorithm of the SMP OS. Flow begins at block 1102.

At block 1102, a timer generates an interrupt request to a VPE 102,which are the exception domains of the microprocessor 100. In oneembodiment, the timer interrupt request is an asynchronous hardwareinterrupt generated by the MIPS PRA Count/Compare register pairs of oneof the VPEs 102 of microprocessor 100, and the Count/Compare registerpairs of the other VPEs 102 are all disabled. Flow proceeds to block1104.

At block 1104, the interrupted VPE 102 selects an eligible threadcontext 104 bound to itself to service the timer interrupt request. Asdescribed above, in the MIPS MT ASE, a thread context 104 is eligible ifits IXMT bit 518 is clear and the curVPE field 558 of the TCBindRegister 556 of FIG. 5K specifies to which VPE 102 the thread context104 is bound. In one embodiment, the method for choosing the eligiblethread context 104 to service an asynchronous exception isimplementation-dependent and may be adapted to satisfy the particularapplication in which the microprocessor 100 is employed. For example,the VPE 102 may select an eligible thread context 104 in a randomfashion. For another example, the VPE 102 may select an eligible threadcontext 104 in a round-robin order. For another example, the VPE 102 mayselect a thread context 104 based on the relative priorities of thethread contexts 104, such as selecting the thread context 104 having thelowest relative instruction issue priority, or a lowest relativepriority for servicing exceptions. Flow proceeds to block 1106.

At block 1106, the VPE 102 suspends execution of the threads executingon all thread contexts 104 bound to the VPE 102 except for the threadcontext 104 selected at block 1104. In particular, the VPE 102 ceases toissue instructions to the execution pipeline of the threads. Flowproceeds to block 1108.

At block 1108, the VPE 102 saves the restart address of the selectedthread context 104 into the EPC Register 598, sets the EXL bit 576 ofthe Status Register 571, and populates the MIPS PRA Cause register, allof the VPE's 102 Coprocessor 0 VPE context 106. Flow proceeds to block1112.

At block 1112, the VPE 102 causes the selected thread context 104 toexecute a general exception handler at the general exception vectoraccording to the MIPS PRA. The general exception handler decodes thecause of the exception via the MIPS PRA Cause register and StatusRegister 571 and determines the exception was an asynchronous hardwareinterrupt generated by the timer. Consequently, the general exceptionhandler calls the timer interrupt service routine, which among otherfunctions, schedules processes according to the preemptive multitaskingalgorithm of the operating system. In one embodiment, the timerinterrupt routine may call a separate routine dedicated to schedulingprocesses. Flow proceeds to block 1114.

At block 1114, the timer interrupt service routine determines whether anew process, or task, should be scheduled on the selected thread context104 according to the SMP OS multitasking scheduling algorithm. If so,the timer interrupt service routine schedules a new process to run onthe selected thread context 104; otherwise, the timer interrupt serviceroutine leaves the current process to run on the selected thread context104. It is noted that a thread and a process herein are not necessarilysynonymous. A process is an entity managed by the SMP operating system,and typically comprises entire programs, such as application programs orportions of the operating system itself; whereas a thread is simply astream of instructions, which of course may be a stream of instructionsof an operating system process, or task. Flow proceeds to block 1116.

At block 1116, the timer interrupt service routine issues asoftware-emulated inter-processor interrupt to each other thread context104 in the microprocessor 100, according to FIG. 10 and/or the sourcecode listing. In particular, if the target thread context 104 is boundto the same VPE 102 as the selected thread context 104 and the targetthread context 104 is not exempted from servicing exceptions (asdetermined by the IXMT bit 518), then the timer interrupt serviceroutine performs a software-emulated inter-processor interrupt to thetarget thread context 104 according to FIG. 10; if the target threadcontext 104 is bound to the same VPE 102 as the selected thread context104 but the target thread context 104 is exempted from servicingexceptions, then the timer interrupt service routine places the timerinterrupt service IPI message on the target thread context's 104 queueat lines 48-62 of the source code; and if the target thread context 104is bound to a different VPE 102 as the selected thread context 104, thenthe timer interrupt service routine will place an IPI message on a queueassociated with the target thread context 104 and issue a MIPS PRAasynchronous software interrupt to the target VPE 102, i.e., to the VPE102 to which the target thread context 104 is bound, according to lines23-28 of the source code, which will cause the queue to be sampled anddrained.

At block 1118, the timer interrupt service routine calls the operatingsystem return from interrupt code, which executes an ERET instruction.If a new process was scheduled to run at block 114, then the ERET causesthe newly scheduled process to run; otherwise, the ERET causes theprocess that was interrupted by the timer interrupt request to continuerunning. Flow proceeds to block 1122.

At block 1122, each thread context 104 that was the target of asoftware-emulated inter-processor interrupt performed at block 1116eventually calls the inter-processor interrupt service routine,according to block 1034 of FIG. 10, after performing the other steps ofFIG. 10. On each thread context 104, the inter-processor interruptservice routine calls the timer interrupt service routine, whichschedules a new process to run on the thread context 104, ifappropriate, similar to the manner described above with respect to block1114. When the inter-processor interrupt handler completes, theoperating system return from interrupt code is called, which executes anERET instruction, according to block 1036 of FIG. 10. If the timerinterrupt service routine scheduled a new process to run on the threadcontext 104, then the newly scheduled process will run on the threadcontext 104 when the return from interrupt code executes the ERET atblock 1036 of FIG. 10, rather than thread B, i.e., rather than thethread that was halted by the software-emulated directed inter-processorinterrupt. If so, thread B will eventually be scheduled to run again sothat it may complete. If the timer interrupt service routine did notschedule a new process to run on the thread context 104, then thread Bwill continue running when the ERET is executed. Flow ends at block1122.

As may be observed from FIG. 11, the software emulation of directedexceptions described according to FIG. 10 enables the SMP OS to treateach thread context as an operating system level CPU, in particular withregard to preemptive process scheduling.

Although the present invention and its objects, features, and advantageshave been described in detail, other embodiments are encompassed by theinvention. For example, although embodiments have been described inwhich the state saved by the initiating thread context comprises therestart address, execution privilege level, interrupt enable state,and/or register set contents of the target thread context, in otherprocessor architectures the initiating thread context may saveadditional thread context state relevant to the particularmultithreading processor architecture. Furthermore, although embodimentshave been described herein with respect to a MIPS MT ASE processor core,the present invention is not limited to such embodiments; rather anymultithreading system that does not provide an architected means of oneMP OS processor to direct an IPI to another MP OS processor, but whichincludes architected instructions that enable one MP OS processor toread and modify the state of the other MP OS processor, may employ thesoftware emulation of directed exceptions invention described herein.Still further, although embodiments have been described herein withrespect to the MIPS32® Architecture, the invention is not limited tosuch embodiments, but may be extended to other architectures, such asthe MIPS64® Architecture and non-MIPS architectures. Finally, althoughembodiments have been described in the context of directed timerinterrupts, the present invention may be employed to emulate viasoftware other types of exceptions directed from one thread context toanother.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample, and not limitation. It will be apparent to persons skilled inthe relevant computer arts that various changes in form and detail canbe made therein without departing from the scope of the invention. Forexample, in addition to using hardware (e.g., within or coupled to aCentral Processing Unit (“CPU”), microprocessor, microcontroller,digital signal processor, processor core, System on Chip (“SOC”), or anyother device), implementations may also be embodied in software (e.g.,computer readable code, program code, instructions and/or data disposedin any form, such as source, object or machine language) disposed, forexample, in a computer usable (e.g., readable) medium configured tostore the software. Such software can enable, for example, the function,fabrication, modeling, simulation, description and/or testing of theapparatus and methods described herein. For example, this can beaccomplished through the use of general programming languages (e.g., C,C++), GDSII databases, hardware description languages (HDL) includingVerilog HDL, VHDL, and so on, or other available programs and databases.Such software can be disposed in any known computer usable medium suchas semiconductor, magnetic disk, or optical disc (e.g., CD-ROM, DVD-ROM,etc.). The software can also be disposed as a computer data signalembodied in a computer usable (e.g., readable) transmission medium(e.g., carrier wave or any other medium including digital, optical, oranalog-based medium). Embodiments of the present invention may includemethods of providing operating system software described herein byproviding the software and subsequently transmitting the software as acomputer data signal over a communication network including the Internetand intranets, such as shown in FIGS. 12 and 13. It is understood thatthe apparatus and method described herein may be included in asemiconductor intellectual property core, such as a microprocessor core(e.g., embodied in HDL) and transformed to hardware in the production ofintegrated circuits. Additionally, the apparatus and methods describedherein may be embodied as a combination of hardware and software. Thus,the present invention should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.  1 voidsmtc_send_ipi(int cpu, int type, unsigned int action)  2 {  3 inttcstatus;  4 smtc_ipi *pipi;  5 long flags;  6 int mtflags;  7  8 if(cpu== smp_processor_id( )) {  9 printk(“Cannot Send IPI to self!\n”);  10return;  11 }  12 /* Set up a descriptor, to be delivered eitherpromptly or queued */  13 pipi = smtc_ipi_dq(&freeIPIq);  14 if(pipi ==NULL) {  15 bust_spinlocks(1);  16 mips_mt_regdump(dvpe( ));  17panic(“IPI Msg. Buffers Depleted\n”);  18 }  19 pipi−>type = type;  20pipi−>arg = (void *)action;  21 pipi−>dest = cpu;  22if(cpu_data[cpu].vpe_id != cpu_data[smp_processor_id( )].vpe_id) {  23/* If not on same VPE, enqueue and send cross-VPE interrupt */  24smtc_ipi_nq(&IPIQ[cpu], pipi);  25 LOCK_CORE_PRA( );  26settc(cpu_data[cpu].tc_id);  27 write_vpe_c0_cause(read_vpe_c0_cause( )| C_SW1);  28 UNLOCK_CORE_PRA( );  29 } else {  30 /*  31  * Notsufficient to do a LOCK_MT_PRA (dmt) here.  32  * since ASID shootdownon the other VPE may  33  * collide with this operation.  34  */  35LOCK_CORE_PRA( );  36 settc(cpu_data[cpu].tc_id);  37 /* Halt thetargeted TC */  38 write_tc_c0_tchalt(TCHALT_H);  39 mips_ihb( );  40 41 /*  42  * Inspect TCStatus - if IXMT is set, we have to queue  43  *a message. Otherwise, we set up the “interrupt”  44  * of the other TC 45  */  46 tcstatus = read_tc_c0_tcstatus( );  47  48 if ((tcstatus &TCSTATUS_IXMT) != 0) {  49 /*  50  * Spin-waiting here can deadlock,  51 * so we queue the message for the target TC.  52  */  53write_tc_c0_tchalt(0);  54 UNLOCK_CORE_PRA( );  55 /* Try to reduceredundant timer interrupt messages */  56 if(type == SMTC_CLOCK_TICK) { 57 if(atomic_postincrement(&ipi_timer_latch[cpu])!=0) {  58smtc_ipi_nq(&freeIPIq, pipi);  59 return;  60 }  61 }  62smtc_ipi_nq(&IPIQ[cpu], pipi);  63 } else {  64 post_direct_ipi(cpu,pipi);  65 write_tc_c0_tchalt(0);  66 UNLOCK_CORE_PRA( );  67 }  68 } 69 }  70 /*  71  * Send IPI message to Halted TC, TargTC already havingbeen set  72  */  73 void post_direct_ipi(int cpu, smtc_ipi *pipi)  74 { 75 struct pt_regs *kstack;  76 unsigned long tcstatus;  77 unsignedlong tcrestart;  78 extern u32 kernelsp[NR_CPUS];  79 extern void__smtc_ipi_vector(void);  80  81 /* Extract Status, EPC from halted TC*/  82 tcstatus = read_tc_c0 tcstatus( );  83 tcrestart =read_tc_c0_tcrestart( );  84 /* If TCRestart indicates a WAITinstruction, advance the PC */  85 if((tcrestart & 0x80000000)  86 &&((*(unsigned int *)tcrestart & 0xfe00003f) == 0x42000020)) {  87tcrestart += 4;  88 }  89 /*  90  * Save on TC's future kernel stack  91 *  92  * CU bit of Status is indicator that TC was  93  * alreadyrunning on a kernel stack...  94  */  95 if(tcstatus & ST0_CU0) {  96 /*Note that this “− 1” is pointer arithmetic */  97 kstack = ((structpt_regs *)read_tc_gpr_sp( )) − 1;  98 } else {  99 kstack = ((structpt_regs *)kernelsp[cpu]) − 1; 100 } 101 102 kstack−>cp0_epc =(long)tcrestart; 103 /* Save TCStatus */ 104 kstack−>cp0_tcstatus =tcstatus; 105 /* Pass token of operation to be performed kernel stackpad area */ 106 kstack−>pad0[4] = (unsigned long)pipi; 107 /* Passaddress of function to be called likewise */ 108 kstack−>pad0[5] =(unsigned long)&ipi_decode; 109 /* Set interrupt exempt and kernel mode*/ 110 tcstatus |= TCSTATUS_IXMT; 111 tcstatus &= ˜TCSTATUS_TKSU; 112write_tc_c0_tcstatus(tcstatus); 113 ehb( ); 114 /* Set TC Restartaddress to be SMTC IPI vector */ 115write_tc_c0_tcrestart(__smtc_ipi_vector); 116 } 117 /* 118  * AssemblyLanguage Function for MIPS MT IPI support 119  */ 120 121 /* 122  *“Software Interrupt” linkage. 123  * 124  * This is invoked when an“Interrupt” is sent from one TC to another, 125  * where the TC to beinterrupted is halted, has it's Restart address 126  * and Status valuessaved by the “remote control” thread, then modified 127  * to causeexecution to begin here, in kernel mode. This code then 128  * disguisesthe TC state as that of an exception and transfers 129  * control to thegeneral exception or vectored interrupt handler. 130  */ 131 132 .setnoreorder 133 134 /* 135 1) Set EXL (this is per-VPE, so this can't bedone by proxy!) 136 2) Restore the K/CU and IXMT bits to the pre“exception” state 137  (EXL means no interrupts and access to the kernelmap). 138 3) Set EPC to be the saved value of TCRestart. 139 4) Jump tothe exception handler entry point passed by the sender. 140 */ 141 142/* 143  * Revised vision: Set EXL and restore K/CU/IXMT state of 144  *pre-halt thread, then save everything and call though 145  * somefunction pointer to imaginary_exception, which 146  * will parse aregister value or memory message queue to 147  * deliver things likeinterprocessor interrupts. On return 148  * from that function, jump tothe global ret_from_irq code 149  * to invoke the scheduler and returnas appropriate. 150  */ 151 152 #define PT_PADSLOT4 (PT_R0-8) 153#define PT_PADSLOT5 (PT_R0-4) 154 155 .text 156 .align 5 157FEXPORT(__smtc_ipi_vector) 158 .set noat 159 /* Disable threadscheduling to make Status update atomic */ 160 dmt k1 161 ehb 162 /* SetEXL */ 163 mfc0 k0,CP0_STATUS 164 ori k0,k0,ST0_EXL 165 mtc0k0,CP0_STATUS 166 ehb 167 /* Thread scheduling now inhibited by EXL.Restore TE state. */ 168 andi k1,k1,VPECONTROL_TE 169 beqz k1,1f 170 emt171 1: 172 /* 173  * The IPI sender has put some information on theanticipated 174  * kernel stack frame. If we were in user mode, thiswill be 175  * built above the saved kernel SP. If we were already inthe 176  * kernel, it will be built above the current CPU SP. 177  * 178 * Were we in kernel mode, as indicated by CU0? 179  */ 180 sl1 k1,k0,3181 .set noreorder 182 bltz k1,2f 183 move k1,sp 184 .set reorder 185 /*186  * If previously in user mode, set CU0 and use kernel stack. 187  */188 li k1,ST0_CU0 189 or k1,k1,k0 190 mtc0 k1,CP0_STATUS 191 ehb 192get_saved_sp 193 /* Interrupting TC will have pre-set values in slots inthe new frame */ 194 2: subu k1,k1,PT_SIZE 195 /* Load TCStatus Value */196 lw k0,PT_TCSTATUS(k1) 197 /* Write it to TCStatus to restoreCU/KSU/IXMT state */ 198 mtc0 k0,$2,1 199 ehb 200 lw k0,PT_EPC(k1) 201mtc0 k0,CP0_EPC 202 /* SAVE_ALL macro will redundantly recompute the SP,but use for now */ 203 SAVE_ALL 204 CLI 205 move a0,sp 206 /* Functionto be invoked passed stack pad slot 5 */ 207 lw t0,PT_PADSLOT5(sp) 208/* Argument from sender passed in stack pad slot 4 */ 209 lwa1,PT_PADSLOT4(sp) 210 jalr t0 211 nop 212 j ret_from_irq 213 nop 214215 /* 216  * Move to kernel mode and disable interrupts. 217  * Set cp0enable bit as sign that we're running on the kernel stack 218  */ 219 .macro CLI 220 #if !defined(CONFIG_MIPS_MT_SMTC) 221  mfc0 t0,CP0_STATUS 222  li t1, ST0_CU0 | 0x1f 223  or t0, t1 224  xori t0, 0x1f225  mtc0 t0, CP0_STATUS 226 #else /* CONFIG_MIPS_MT_SMTC */ 227  /* 228  * For SMTC, we need to set privilege 229   * and disable interruptsonly for the 230   * current TC, using the TCStatus register. 231   */232  .set mips32 233  mfc0 t0,CP0_TCSTATUS 234  /* Fortunately CU 0 isin the same place in both registers */ 235  /* Set TCU0, TMX, TKSU (forlater inversion) and IXMT */ 236  li t1, ST0_CU0 | 0x08001c00 237  ort0,t1 238  /* Clear TKSU, leave IXMT */ 239  xori t0, 0x00001800 240 mtc0 t0, CP0_TCSTATUS 241  ehb 242  .set mips0 243  /* We need to leavethe global IE bit set, but clear EXL...*/ 244  mfc0 t0, CP0_STATUS 245 ori t0, ST0_EXL | ST0_ERL 246  xori t0, ST0_EXL | ST0_ERL 247  mtc0 t0,CP0_STATUS 248 #endif /* CONFIG_MIPS_MT_SMTC */ 249  irq_disable_hazard250  .endm

1. A method for performing preemptive scheduling of a plurality ofprocesses managed by a symmetric multiprocessor operating system on amultithreading microprocessor, the method comprising: receiving, by anexception domain, a periodic interrupt request from a timer, theexception domain comprising a plurality of thread contexts of themicroprocessor; executing a first interrupt handler on one of theplurality of thread contexts to service the interrupt request, saidexecuting comprising: scheduling a new process to execute on the onethread context; and writing an address of a second interrupt handler toa restart address register of each thread context other than the one ofthe plurality of thread contexts; and scheduling, by the secondinterrupt handler, a respective new process to execute on each threadcontext other than the one of the plurality of thread contexts.
 2. Themethod as recited in claim 1, wherein said executing the first interrupthandler further comprises: for each thread context other than the one ofthe plurality of thread contexts, writing to a control register of thethread context to commence execution of the second interrupt handlerthereon, prior to said scheduling by the second interrupt handler therespective new process to execute on the thread context.
 3. The methodas recited in claim 2, wherein said executing the first interrupthandler further comprises: for each thread context other than the one ofthe plurality of thread contexts, writing to the control register of thethread context to halt execution of a thread executing thereon, prior tosaid writing the address of the second interrupt handler to the restartaddress register.
 4. The method as recited in claim 3, wherein saidexecuting the first interrupt handler further comprises: for each threadcontext other than the one of the plurality of thread contexts, readinga restart address of the thread from the restart address register, priorto said writing the address of the second interrupt handler to therestart address register.
 5. The method as recited in claim 4, whereinsaid executing the first interrupt handler further comprises: for eachthread context other than the one of the plurality of thread contexts,saving the restart address to a memory coupled to the microprocessor,after said reading the restart address of the thread from the restartaddress register.
 6. The method as recited in claim 3, wherein saidexecuting the first interrupt handler further comprises: for each threadcontext other than the one of the plurality of thread contexts, readingan execution privilege level of the thread context and saving theexecution privilege level to a memory coupled to the microprocessor,after said writing to the control register of the thread context to haltexecution of the thread executing thereon, and prior to said writing tothe control register of the thread context to commence execution of thesecond interrupt handler thereon.
 7. The method as recited in claim 1,further comprising: selecting the one thread context to service thetimer interrupt request, in response to said receiving the timerinterrupt request, and prior to said executing the first interrupthandler on the one thread context.
 8. The method as recited in claim 7,wherein said selecting the one thread context to service the interruptrequest comprises selecting an eligible one of the plurality of threadcontexts, wherein one of the plurality of thread contexts is eligible ifit is enabled to service exceptions.
 9. The method as recited in claim8, wherein said selecting the one thread context to service theinterrupt request comprises selecting an eligible one of the pluralityof thread contexts in a pseudo-random fashion.
 10. The method as recitedin claim 8, wherein said selecting the one thread context to service theinterrupt request comprises selecting an eligible one of the pluralityof thread contexts in a round-robin fashion.
 11. The method as recitedin claim 8, wherein said selecting the one thread context to service theinterrupt request comprises selecting an eligible one of the pluralityof thread contexts based on relative priorities of the plurality ofthread contexts.
 12. The method as recited in claim 7, furthercomprising: suspending execution of the plurality of thread contextsother than the selected one thread context, after said selecting the onethread context.
 13. The method as recited in claim 1, wherein the timerinterrupt request is non-specific about which of the plurality of threadcontexts is to be interrupted to execute the first interrupt handler toservice the request.
 14. The method as recited in claim 1, wherein thetimer interrupt request is non-directable specifically to any of theplurality of thread contexts.
 15. The method as recited in claim 1,wherein the plurality of thread contexts are bound to the exceptiondomain.
 16. The method as recited in claim 1, wherein the microprocessorcomprises a second exception domain comprising a second plurality ofthread contexts each bound to the second exception domain, wherein saidexecuting the first interrupt handler further comprises: for each of thesecond plurality of thread contexts, placing a message on a messagequeue to request the thread context to service the request.
 17. Themethod as recited in claim 16, wherein said executing the firstinterrupt handler further comprises: interrupting the second exceptiondomain, after said placing the messages.
 18. The method as recited inclaim 17, wherein said interrupting the second exception domaincomprises: writing to a control register of the second exception domainto cause a software interrupt.
 19. The method as recited in claim 1,wherein said executing the first interrupt handler further comprises:for each thread context other than the one of the plurality of threadcontexts: reading a status register of the thread context to determinewhether the thread context is exempted from servicing exceptions; andplacing a message on a message queue to request the thread context toservice the request, if the status register indicates the thread contextis exempted from servicing exceptions, rather than said writing theaddress of the second interrupt handler to the restart address registerof the thread context.
 20. The method as recited in claim 1, whereinsaid executing the first interrupt handler further comprises: for eachthread context other than the one of the plurality of thread contexts,saving a current state of the thread context, prior to said writing tothe restart address register.
 21. A multiprocessor computer system forperforming preemptive scheduling of a plurality of processes managed bya symmetric multiprocessor operating system (SMP OS) running on thesystem, the system comprising: a multithreading microprocessor,comprising: an exception domain, comprising a plurality of threadcontexts each comprising a restart address register; and a timer,configured to generate a periodic interrupt request to said exceptiondomain, wherein said exception domain is configured to select aneligible one of said plurality of thread contexts to service saidinterrupt request, wherein said timer interrupt request is non-specificregarding which of said plurality of thread contexts to select toservice said request; and a memory, coupled to said microprocessor, forstoring a first interrupt handler of the SMP OS for execution on saidselected thread context to service said interrupt request, wherein saidfirst interrupt handler is configured to: schedule a set of processesassigned by the SMP OS for execution on said selected thread context;and write an address of a second interrupt handler to said restartaddress register of each of said plurality of thread contexts other thansaid selected thread context, wherein said second interrupt handler isconfigured to schedule a plurality of sets of processes assigned by theSMP OS for execution on respective ones of said plurality of threadcontexts other than said selected thread context.
 22. The system asrecited in claim 21, wherein said first interrupt handler is furtherconfigured to, for each thread context other than said selected threadcontext, write to a control register of said thread context to commenceexecution of said second interrupt handler thereon, prior to said secondinterrupt handler scheduling said plurality of sets of processes. 23.The system as recited in claim 22, wherein said first interrupt handleris further configured to, for each thread context other than saidselected thread context, write to said control register of said threadcontext to halt execution of a thread executing thereon, prior towriting said address of said second interrupt handler to said restartaddress register.
 24. The system as recited in claim 23, wherein saidfirst interrupt handler is further configured to, for each threadcontext other than said selected thread context, read a restart addressof said thread from said restart address register, prior to writing saidaddress of said second interrupt handler to said restart addressregister.
 25. The system as recited in claim 24, wherein said firstinterrupt handler is further configured to, for each thread contextother than said selected thread context, save said restart address to amemory coupled to said microprocessor, prior to reading said restartaddress of said thread from said restart address register.
 26. Thesystem as recited in claim 23, wherein said first interrupt handler isfurther configured to, for each thread context other than said selectedthread context, read an execution privilege level of said thread contextand save said execution privilege level to a memory coupled to saidmicroprocessor, subsequent to writing to said control register of saidthread context to halt execution of said thread executing thereon, andprior to writing to said control register of said thread context tocommence execution of said second interrupt handler thereon.
 27. Thesystem as recited in claim 22, wherein said control register comprises aMIPS Multithreading Application Specific Extension (MT ASE) TCHaltregister.
 28. The system as recited in claim 21, wherein said exceptiondomain is further configured to suspend execution of said plurality ofthread contexts other than said selected thread context, in response toreceiving said timer interrupt request.
 29. The system as recited inclaim 21, wherein said microprocessor further comprises: a secondexception domain, comprising a second plurality of thread contexts,wherein said first interrupt handler is further configured to, for eachof said second plurality of thread contexts, place a message on amessage queue to request said thread context to service said request.30. The system as recited in claim 29, wherein said first interrupthandler is further configured to interrupt said second exception domain,after placing said messages.
 31. The system as recited in claim 30,wherein said first interrupt handler writes to a control register ofsaid second exception domain to cause a software interrupt.
 32. Thesystem as recited in claim 31, wherein said control register comprises aMIPS Privileged Resource Architecture (MT PRA) Cause register.
 33. Thesystem as recited in claim 21, wherein said first interrupt handler isfurther configured to, for each thread context other than said selectedthread context, read a status register of said thread context todetermine whether said thread context is exempted from servicingexceptions, and place a message on a message queue to request saidthread context to service said request, if said status registerindicates said thread context is exempted from servicing exception,rather than writing said address of said second interrupt handler tosaid restart address register of said thread context.
 34. The system asrecited in claim 33, wherein said status register comprises a MIPSMultithreading Application Specific Extension (MT ASE) TCStatusregister.
 35. The system as recited in claim 21, wherein said firstinterrupt handler is further configured to, for each thread contextother than said selected thread context, save a current state of saidthread context, prior to writing to said restart address register. 36.The system as recited in claim 21, wherein said restart address registercomprises a MIPS Multithreading Application Specific Extension (MT ASE)TCRestart register.
 37. The system as recited in claim 21, wherein saidplurality of thread contexts comprise MIPS Application SpecificExtension (MT ASE) thread contexts.
 38. A computer program product foruse with a computing device, the computer program product comprising: acomputer usable medium, having computer readable program code embodiedin said medium, for causing a method for performing preemptivescheduling of a plurality of processes managed by a symmetricmultiprocessor operating system on a multithreading microprocessor, saidcomputer readable program code comprising: first program code forproviding a step of receiving, by an exception domain, a periodicinterrupt request from a timer, the exception domain comprising aplurality of thread contexts of the microprocessor; second program codefor providing a step of executing a first interrupt handler on one ofthe plurality of thread contexts to service the interrupt request, saidexecuting comprising: scheduling a new process to execute on the onethread context; and writing an address of a second interrupt handler toa restart address register of each thread context other than the one ofthe plurality of thread contexts; and third program code for providing astep of scheduling, by the second interrupt handler, a respective newprocess to execute on each thread context other than the one of theplurality of thread contexts.
 39. The computer program product of claim38, wherein said computer readable program code further comprises:fourth program code for providing a step of for each thread contextother than the one of the plurality of thread contexts, writing to acontrol register of the thread context to commence execution of thesecond interrupt handler thereon, prior to said scheduling by the secondinterrupt handler the respective new process to execute on the threadcontext.
 40. The computer program product of claim 39, wherein saidcomputer readable program code further comprises: fifth program code forproviding a step of for each thread context other than the one of theplurality of thread contexts, writing to the control register of thethread context to halt execution of a thread executing thereon, prior tosaid writing the address of the second interrupt handler to the restartaddress register.
 41. A method for providing operating system softwarefor performing preemptive scheduling of a plurality of processes managedby a symmetric multiprocessor operating system on a multithreadingmicroprocessor, the method comprising: providing computer-readableprogram code describing the operating system software, the program codecomprising: first program code for providing a step of receiving, by anexception domain, a periodic interrupt request from a timer, theexception domain comprising a plurality of thread contexts of themicroprocessor; second program code for providing a step of executing afirst interrupt handler on one of the plurality of thread contexts toservice the interrupt request, said executing comprising: scheduling anew process to execute on the one thread context; and writing an addressof a second interrupt handler to a restart address register of eachthread context other than the one of the plurality of thread contexts;and third program code for providing a step of scheduling, by the secondinterrupt handler, a respective new process to execute on each threadcontext other than the one of the plurality of thread contexts; andtransmitting the computer-readable program code as a computer datasignal on a network.